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Nonreciprocal photonic management can shift the absorption-emission balance in favor of absorption and enhance the conversion efficiency beyond the detailed balance Shockley - Queisser limit. Nonreciprocal photovoltaic (PV) cells can provide the conversion of the entire exergy (Helmholtz free energy) of quasi-monochromatic radiation into electric power. Recent discoveries in electromagnetics have demonstrated the ability to break Kirchhoff’s reciprocity in a variety of ways. The absorption-emission nonreciprocity may be realized via dissipationless one-way optical components as well as via the greenhouse-type electron-photon kinetics that traps the low-energy near-bandgap photons in the cell. We calculate the limiting performance of the nonreciprocal dissipationless monochromatic converter and discuss the limiting efficiency of the nonreciprocal converter based on the greenhouse effect. We also perform detailed modeling of the greenhouse effect in the GaAs PV converter and determined its PV performance for conversion of 809 nm laser radiation. In perovskite PV cells the greenhouse filter establishes a sharp absorption edge and reduces conversion losses related to the distributed PV bandgap and laser-cell matching losses.

Recent progress in semiconductor materials with minor nonradiative recombination has stimulated investigations of novel photovoltaic (PV) converters with optical control of radiative emission. Angle restricted emission was experimentally demonstrated in PV devices with external photon recycling due to specific photonic crystals or mirrors. In this work we investigate the power beam conversion by the cell with front “greenhouse” filter, which transmits the laser light, but recycles the low energy bandgap quanta emitted by the semiconductor cell. We calculate the limiting characteristics of the greenhouse PV converters and optimize design of the converter taking into account the nonradiative recombination processes. In optimized devices addition of the greenhouse filter can increase power beam conversion efficiency by several percent.

Having been inspired by the tremendous progress in material nanoscience and device nanoengineering, hybrid phototransistors combine solution processed colloidal semiconductor quantum dots (QDs) with graphene or two-dimensional (2D) semiconductor materials. Novel detectors demonstrate ultrahigh photoconductive gain, high and selective photoresponse, low noise, and very high responsivity in visible- and near-infrared ranges. The outstanding performance of phototransistors is primarily due to the strong, selective, and size tunable absorption of QDs and fast charge transfer in 2D high mobility conductors. However, the relatively small mobility of QD nanomaterials was a technological barrier, which limited the operating rate of devices. Very recent innovations in detector design and significant progress in QD ligand engineering provide effective tools for further qualitative improvements. This article reviews the recent progress in material science, nanophysics, and device engineering related to hybrid phototransistors. Detectors based on various QD nanomaterials and several 2D conductors are compared, and advantages and disadvantages of various nanomaterials for applications in hybrid phototransistors are identified. We also benchmark the experimental characteristics with model results that establish interrelations and tradeoffs between detector characteristics, such as responsivity, dark and noise currents, the photocarrier lifetime, response, and noise bandwidths. We have shown that the most recent phototransistors demonstrate performance limited by the fundamental generation recombination noise in high gain devices. Interrelation between the dynamic range of the detector and the detector sensitivity is discussed. The review is concluded with a brief discussion of the remaining challenges and possible significant improvements in the performance of hybrid phototransistors.

In0.85Ga0.15As photodetectors grown on GaAs substrates using an interfacial misfit array-based simple buffer are studied. The material quality is assessed with a range of characterization tools showing low surface roughness and low density of threading dislocations. These results indicate a significant improvement on crystal quality compared to structures grown on InP substrates by using metamorphic buffers. Quantum efficiency and responsivity measurements show good performance of the fabricated devices between 1.5 and 2.5 µm, making them highly suitable for short-wavelength infrared applications.

Novel approach to optimize quantum dot (QD) materials for specific optoelectronic applications is based on engineering of nanoscale potential profile, which is created by charged QDs. The nanoscale barriers prevent capture of photocarriers and drastically increase the photoelectron lifetime, which in turn strongly improves the photoconductive gain, responsivity, and sensitivity of photodetectors and decreases the nonradiative recombination losses of photovoltaic devices. QD charging may be created by various types of selective doping. To investigate effects of selective doping, we model, fabricated, and characterized AlGaAs/InAs QD structures with n-doping of QD layers, doping of interdot layers, and bipolar doping, which combines p-doping of QD layers with strong n-doping of the interdot space. We have measured spectral characteristics of photoresponse, photocurrent and dark current. The experimental data show that providing the same electron population of QDs, the bipolar doping creates the most contrasting nanoscale profile with the highest barriers around dots.

We demonstrate the integration of subwavelength moth-eye antireflection nanostructures onto a fully packaged commercial eight cell array of dual junction epitaxial lift-off GaAs solar cells. A low cost, scalable, and large area nanosphere lithography patterning process was used to fabricate the nanostructures on a polymer PET packaging layer. The mechanically flexible array shows 29.80% PCE at normal incidence and enhanced J sc at large angles of incidence due to the anti-reflection nanostructures.

We demonstrate the integration of subwavelength moth-eye and Al2O3 nanoisland structures fabricated on polymer packaging sheets and the surface of conventional Al2O3/TiO2 bilayer antireflection coatings, respectively, with epitaxial lift-off single-junction GaAs solar cells. The mechanically flexible cell structure with the integrated optical nanostructures shows substantially improved photovoltaic performance under various incident angles and bending radii compared to devices without such structures: the increase in short-circuit current density arising from integration of these nanostructures ranges from 9% at normal incidence to 52% at 80° incidence; and the reduction in short-circuit current density under moderate bending decreases from 9.7% to 6.7%.

We report a significant broadband enhancement of the external quantum efficiency of the quantum dot solar cell by coupling with plasmonic nanostars via a simple and scalable “boiling deposition” technique. The multispiked nanostars provide broadband scattering and absorption cross-sections, which can be engineered to dramatically boost the performance of the solar cells. The localized near field modes of nanostars result in an external quantum efficiency enhancement over 400% for short-wavelength light absorbed in the emitter, while plasmon light scattering causes distinct improvement in quantum efficiency (10–50%) in the long-wavelength region up to 1100 nm. Finite difference time domain method is adopted to explain the origin of the optical absorption enhancement in the quantum dot solar cells. The broadband light concentration by plasmonic nanostars can significantly reduce the amount of quantum dot materials required for a solar cell and provide efficient utilization of the full solar spectrum.

Quantum dot infrared photodetectors (QDIPs) have attracted significant attention due to selective photoresponse, high photoconductive gain, and numerous possibilities for nanoscale engineering of photoelectron processes, which control the detector characteristics. Our approach to improving QDIP performance is based on optimization of three dimensional nanoscale potential profile created by charged quantum dots (QDs). Nanoscale profile around QDs allows us to control photoelectron capture processes, which determines the photoelectron lifetime, detector operating speed, responsivity, the spectral density of noise, noise bandwidth, and the detector dynamic range. The nanoscale potential profile is determined by doping of QDs and inter-dot space. In this work, we study various ways of selective doping and its effects on characteristics of photodetectors. We investigate and compare intra-dot doping, inter-dot doping, and complex bipolar doping. To investigate effects of selective doping, we fabricated AlGaAs/InAs QD structures with n-doping of QD layers, structures with n-doping of barriers, and structures with p-doping of QD layers and n-doping of interdot space. We measured dark current, spectral photoresponse, voltage dependence of responsivity, and noise characteristic. The photoresponse is improved due to photon-electron coupling, which increases with QD filling by electrons. However, the noise current also increases due to increase in QD filling. Therefore, possibilities for improvement of QDIP structures with unipolar doping are very limited. Our results show that spectral photoresponse, responsivity, and detector sensitivity are substantially improved due to bipolar doping, which provides decoupled control of electron filling of QDs and the potential barriers around QDs.

Reduction in surface and interface reflectance via the integration of subwavelength nanostructures in flexible polymer packaging material combined with incorporation of dielectric nanoislands into a conventional two-layer antireflection coating has been demonstrated, analyzed and optimized. Transmittance measurements of moth-eye textured polymer packaging sheets with different tapered pillar heights fabricated by reactive-ion etching and nanosphere lithography provide insights into the choice of the optimum nanostructure dimensions. Detailed computational modeling and simulations elucidate the physical nature of the antireflection performance of dielectric nanoisland structures integrated with a commercial two-layer antireflection coating, and provide guidance for design of the nanoisland structure for optimum antireflection performance. Measurements show that the integration of appropriately designed nanostructures in both polymer packaging material and conventional antireflection layers enables substantial increases in external quantum efficiency (E.Q.E.) and short-circuit current density (Jsc) over a broad range of incident angles, compared to structures with conventional bilayer antireflection coatings and unpatterned polymer packaging sheets. A 1.1× increase in Jsc, derived directly from E.Q.E. measurements, at normal incidence, increasing to 1.67× improvement at 80° angle of incidence, suggests that such an approach is promising for a variety of photovoltaic applications, particularly those where solar tracking is not feasible or practical. Copyright © 2014 John Wiley & Sons, Ltd.

We present theoretical and experimental analysis of photocarrier kinetics in quantum dot (QD) solar cells. The measurements of the J–V characteristics reveal strong effects of QD charging by selective doping of the interdot space on the solar cell characteristics. We demonstrate that charging of QDs significantly increases electron coupling to sub-bandgap photons, provides effective harvesting of IR energy, and serve as an effective tool for manipulating the potential profile at the micro- and nanoscale. The potential well for electrons in InAs QDs is substantially deeper than that for holes and, due to major differences between the effective masses of electrons and holes, the electron level spacing is substantially larger than the level spacing for holes. Therefore, QDs act as deep traps for electrons but shallow traps for holes. Filling of QDs under illumination is determined by a condition of equality of electron and hole capture rates which is realized via strong exponential dependence of the capture rates on the potential barrier around a charged dot. Without adequate doping of the QD medium, QDs are filled by electrons from the n-doped junction area and deteriorate the solar cell performance. However, selective n-doping of the QD medium results in micro- and nanoscale potential profiles favorable for photovoltaic conversion. Potential barriers around charged QDs decrease the photoelectron capture processes and suppress recombination processes via QDs. The filling of QDs predominantly from dopants in the QD medium allows one to maintain the microscale potential profile analogous to that in the best conventional single-junction solar cells.

GaAs nano-mounds formed by droplet epitaxy are used as templates for growth of self-assembled InAs quantum dot clusters (QDCs). These QDCs are found to contain an average of thirteen dots per cluster, of which there are two families of different sized quantum dots. Excitation intensity-dependent photoluminescence (PL) demonstrates that there is no lateral coupling between the two different size quantum dots. Lateral transfer of carriers is observed between different size quantum dots due to thermal activation as seen in their different temperature-dependent optical behaviors.

We demonstrate the formation of hybrid molecules composed of a pair of a metal particle and a semiconductor quantum ring (QR) by using molecular beam epitaxy on a GaAs (100) surface. To form three-dimensional semiconductor InGaAs QRs with a hole in the center of the structure, a thin layer (10 monolayer) of GaAs was applied on InAs quantum dots, which transformed the distribution of surface free energy. These InGaAs QRs were then used as a template for the localization of Ga metal particles; there was one metal particle on a QR. By choosing the optimal surface temperature (most favorable thermal energy), the surface diffusion of Ga adatoms was effectively enhanced. This, in turn, allowed the localization of Ga metal particles on InGaAs QRs, consequently forming hybrid molecules. This study helps understand the formation of various types of hybrid molecules that will be used in applications in optoelectronics.

Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized.Copyright © 2016 by ASME

We designed, fabricated, and characterized multi-color IR photodetectors with asymmetrical doping of GaAs/AlGaAs double quantum wells (DQW). We measured and analyzed spectral and noise characteristics to evaluate feasibility of these photodetectors for remote temperature sensing at liquid nitrogen temperatures. The bias voltage controls the charge distribution between the two wells in a DQW unit and provides effective tuning of IR induced electron transitions. We have found that the responsivity of our devices is symmetrical and weakly dependent on the bias voltage because the doping asymmetry compensates the effect of dopant migration in the growth direction. At the same time, the asymmetrical doping strongly enhances the selectivity and tunability of spectral characteristics by bias voltage. Multicolor detection of our QWIP is realized by varying the bias voltage. Maximum detection wavelength moves from 7.5 μm to 11.1 μm by switching applied bias from -5 V to 4 V. Modeling shows significant dependence of the photocurrent ratio on the object temperature regardless of its emissivity and geometrical factors. We also experimentally investigated the feasibility of our devices for remote temperature sensing by measuring the photocurrent as a response to blackbody radiation with the temperature from 300°C to 1000°C in the range of bias voltages from -5 V to 5 V. The agreement between modelling and experimental results demonstrates that our QWIP based on asymmetrically doped GaAs/AlGaAs DQW nanomaterial is capable of remote temperature sensing. By optimizing the physical design and varying the doping level of quantum wells, we can generalize this approach to higher temperature measurements. In addition, continuous variation of bias voltage provides fast collection of large amounts of photocurrent data at various biases and improves the accuracy of remote temperature measurements via appropriate algorithm of signal processing.

InAs/GaAs heterostructures have been simultaneously grown by molecular beam epitaxy on GaAs (100), GaAs (100) with a 2° misorientation angle towards [01−1], and GaAs (n 11)B (n = 9, 7, 5) substrates. While the substrate misorientation angle increased from 0° to 15.8°, a clear evolution from quantum dots to quantum well was evident by the surface morphology, the photoluminescence, and the time-resolved photoluminescence, respectively. This evolution revealed an increased critical thickness and a delayed formation of InAs quantum dots as the surface orientation departed from GaAs (100), which was explained by the thermal-equilibrium model due to the less efficient of strain relaxation on misoriented substrate surfaces.

The radiative limit model, based on the black body theory extended to semiconductors and the flow equilibrium in the cell, has been adapted for Ga(x)In(1-x)As thermophotovoltaic devices. The impact of the thermal emitter temperature and the incident power density on the performance of cells for different Ga/In ratios has been investigated. The effects of the thickness of the cell and of light trapping have been investigated as well. A theoretical maximum efficiency of 24.2% has been calculated for a dislocation-free 5-μm-thick cell with a 0.43 eV bandgap illuminated by a source at 1800 K. The model also takes into account Auger recombinations and threading dislocations-related Shockley-Read-Hall recombinations.

Structures containing multiple layers of self-assembled InAs quantum dot clusters within a GaAs matrix are investigated on GaAs (1 0 0) by atomic force microscopy. Droplet homoepitaxy (GaAs nanostructures on planar GaAs) is used to create tiny GaAs nano-mound templates elongated along [0 1 −1]. Due to a high density of monolayer steps on the edge of nano-mounds, deposited InAs prefer to form in clusters around the nano-mound templates. By varying the subsequent InAs monolayer coverages and growth temperatures, two distinctive sizes of quantum dots (QDs) are formed around the nano-mounds for the layer stacking. With a fixed GaAs barrier thickness (10 nm) in between the layers, the resulting QDs from the stacked layers show significant improvement in their size uniformity.

New morphologies of InAs quantum dot (QD) ensembles forming on self-assembled GaAs nano-holed island templates are demonstrated. Droplet homoepitaxy (GaAs/GaAs) is used to generate holed nanoscale-sized mounds that appear to elongate along . Depending on the InAs monolayer (ML) coverages, subsequent InAs deposition forms different sizes and shapes of QD ensembles. While we initially observe the formation of the QDs at the hole sites when less InAs is deposited, QDs form around the edges of the mounds with greater InAs deposition. By varying the InAs depositions and growth temperatures, we demonstrate an ability to control the size and density of QDs. The observed decrease in the necessary critical thickness required for the InAs 2D–3D transition may be due to the higher density of monolayer steps on the sidewalls of the holes and on the edges of the mounds. This hybrid growth approach overcomes some limitations of typical QD growth on planar GaAs surfaces and may find applications in optoelectronics.

State University of New York, Buffalo, New York, USA U.S. Army Research Laboratory, Adelphi, Maryland 20783, USA SUNY Polytechnic Institute, Albany, New York, USA Memory R&D Division, SK Hynix, Icheon-si, Gyeonggi-do, 467-701, Korea Center for Micro- and Nanostructures, TU Vienna, Vienna 1040, Austria

Nanoscale engineering of band profile and potential profile provide effective tools for the management of photoelectron processes in quantum dot (QD) photovoltaic devices. We investigate the QD devices with various 1-μm InAs /GaAs QD media placed in a 3-μm base GaAs p-n junction. We found that n-charging of quantum dots (QDs) create potential barriers around QDs. QD growth between ultrathin AlGaAs layers leads to the formation of AlGaAs “fence” barriers, and reduces the wetting layers (WLs). The barriers around QDs and reduction of the wetting layer substantially suppress recombination processes via QDs. The n-doping of interdot space in QD media enhances electron extraction from QDs. All of our QD devices show short-circuit current, J SC , higher than that of the reference cell, but smaller open-circuit voltage, V OC .. In the QD devices, the short circuit currents increase by ~0.1 mA/cm 2 per dot layer. J SC reaches 28.4 mA/cm 2 in the device with QD media that combines dot charging, fence barriers, and WL reduction.

Efficient photovoltaic energy harvesting requires device structures capable of absorbing a wide spectrum of incident radiation and extracting the photogenerated carriers at high voltages. In this paper, we review the impact of active layer thickness on the voltage performance of GaAs-based photovoltaic device structures. We observe that thin absorber structures can be leveraged to increase the operating voltage of energy harvesting devices. Thin absorbers in combination with advanced light trapping structures provide an exciting pathway for enhancing the performance of flexible, lightweight photovoltaic modules suitable for mobile and portable power applications.

The hybrid structure of GaAs/GaAsSb quantum well (QW)/InAs quantum dots solar cells (QDSCs) is analyzed using power-dependent and temperature-dependent photoluminescence. We demonstrate that placing the GaAsSb QW beneath the QDs forms type-II characteristics that initiate at 12% Sb composition. Current density-voltage measurements demonstrate a decrease in power efficiency with increasing Sb composition. This could be attributed to increased valence band potential in the GaAsSb QW that subsequently limits hole transportation in the QD region. To reduce the confinement energy barrier, a 2 nm GaAs wall is inserted between GaAsSb QW and InAs QDs, leading to a 23% improvement in power efficiency for QDSCs.

The use of subwavelength-scale dielectric nanostructures for both reduced surface and interface reflectance and long-wavelength light trapping in III/V thin-film solar cells is demonstrated and characterized. Antireflection coatings incorporating submicron pyramidal “nanoislands” are fabricated on GaAs thin-film solar cells using a low-cost nanosphere lithography (NSL) process, and shown to improve external quantum efficiency (E.Q.E.) compared to that achieved using a thin-film planar Al 2 O 3 /TiO 2 antireflection coating, with the largest improvements occurring for large angles of incidence. Such structures are also shown to enable, simultaneously, reduced surface reflectance and long-wavelength light trapping, as demonstrated in GaAs/InGaAs quantum well solar cells. Finally, the simultaneous use of subwavelength “moth-eye” structures fabricated in polymer packaging material using NSL and solar cell antireflection structures incorporating dielectric “nanoislands” is demonstrated and analyzed, enabling increases in short-circuit current density of ∼1.1× to 1.67×, depending on angle of incidence, compared to structures using conventional two-layer thin-film antireflection coatings and unpatterned polymer packaging.

The structural complexity of GaAs quantum-dot pairs has been revealed by cross-sectional transmission electron microscopy. As a result of high-temperature droplet epitaxy, the AlGaAs substrate beneath the quantum-dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum-dot pairs are immersed into the substrate and further confined by the re-distributed AlGaAs materials above the substrate plane. There are two underlying mechanisms responsible for the final nanostructure configuration, melt-back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Schematic representation of the preparation of single-component macromolecular capsules by using solid core and mesoporous shell (SC/MS) silica particles as templates. The process involves the infiltration of polyelectrolyte or polymer-drug conjugates into mesoporous shells of SC/MS particles (step 1), followed by crosslinking of the infiltrated polymer chains (step 2) and subsequent removal of the SC/MS silica template (step 3), leading to thick-walled polyelectrolyte or drug-conjugated polymer nanocapsules

The miniaturization of components for the construction of useful devices is an essential feature of modern technology. Their miniaturization permits the assembly of ultra-densely integrated circuits and faster processors. However, along with the developing of silicon-based electronics, it is becoming apparent that intrinsic limitations will prevent their miniaturization down to the nanoscale. To solve these problems, an alternative and promising strategy, called the bottom-up approach, was suggested by an eminent physicist and visionary, Richard Feynman, in 1959. In the bottom-up approach, one can build nanodevices starting from atom or molecules. Via this strategy, a series of significant advances have been achieved in recent years. However many problems still exist, hampering its further development. This said, the researchers were faced with a puzzling problem—How can nanoelectrodes with a controllable gap size be fabricated? This is particularly important because the fundamental basis of molecular electronics requires the electrodes to be fabricated with a gap size commensurate to the size of molecules of interest. Despite reports on successful attempts such as break junction, electrochemical method, and nanowire lithography, the precise control of the gap size still need be resolved. For instance, it is a problem to provide a real time characterization during the fabrication of the nanoelectrodes; thus, the exact gap size is usually undetectable, leaving the precise control of the gap size unfeasible and inefficient. Moreover, the existing methods often are far routine, low yielding and difficult to implement. To solve these problems, Chinese scientists have demonstrated a new method based on the electron-beam-induced deposition (EBID) process to realize a real time and in situ characterization in nanoelectrode fabrication. This technique has thus far been successful in easily and precisely controlling the gap size of the nanoelectrodes. “The research of molecular electronics was launched in 1974, when Ari Aviram and Mark A. Ratner proposed an electrical rectifier by a single molecule with suitable electronic asymmetry. From that time, the fabrication of nanoelectrodes with a molecular gap size remains a puzzle for the researchers. This is also the first obstacle we encountered.” Prof. Yunqi Liu explains, “We tried many methods; however, the present methods are too fastidious for us to implement. Most important, we need a real time and in situ characterization in the fabrication for controlling the gap size of nanoelectrodes; however, the present methods could not afford.” “EBID is a maskless process using a high-intensity electron beam to deposit nanoscale structures on a scanned surface, and it has been widely used in nanofabrication.” Says Prof. Liu “In the scanning electron microscopy (SEM) test of carbon nanotubes (CNTs), we found that the CNTs became broader after electron beam irradiation, and this should originate from EBID. Based on this finding, we developed a new method to produce nanoelectrodes.” Yunqi Liu, the Professor of Institute of Chemistry at the Chinese Academy of Sciences in Beijing, P. R. China, developed the method along with graduate student Dacheng Wei. This work has been published in the May 23, 2008 online edition in Nano Letters (“Real time and in situ control of the gap size of nanoelectrodes for molecular devices). “We place a CNT between Au/Ti electrodes on a SiO2/Si wafer, and then cut it at the middle to form a wide original gap in the range of 10–60 nm. The electrode is exposed to organic vapor to absorb organic molecules on the CNT.” Prof. Liu describes the process, “if we place the electrode in SEM and focus a high-density electron beam on the area of the gap of the electrode, the irradiated part of the CNT will gradually become broader, and as a result the gap becomes narrower. Because this process is observed in real time and in situ by SEM, we can stop the process at any time, and then an electrode with the gap size corresponding to our need is obtained” (Fig. ​(Fig.11). Figure 1 Schematic diagram of the process of the fabrication of a CNT electrode with a controlled nanogap. (a) Bridging a CNT between Au/Ti electrodes. (b) Cutting the CNT by current breakdown method. (c) Adsorbing organic molecules on or in the CNT. (d) Irradiating ... Juxtapose to existing methods, the method proposed by Prof. Liu’s group is very simple and controllable. “What we need is a SEM. In previous research, the SEM serves primarily as a tool to precisely characterize the gap size of the nanoelectrodes. In our method, the SEM plays two roles. First, the SEM provides an in situ and real time characterization of the gap size. Second, the electron beam of SEM induces broadening of CNTs and narrowing of the gap. Now it is very simple for us to fabricate nanoelectrodes with certain gap size. We can fabricate nanoelectrodes with a series of gap sizes.” Prof. Liu says, “moreover, It is a clean process without introducing impure atoms and a nondestructive process for CNT electrodes” (Fig. ​(Fig.22). Figure 2 SEM images of the CNT nanoelectrodes. (a) SEM images of a CNT electrode in the EBID process: (1) just after current breakdown; (2–5) after an EBID process of 2, 4, 6, and 10 min, respectively. (b) CNT electrodes with a series of gap sizes fabricated ... The nanoelectrodes produced by this method have a π-conjugated surface. Prof. Liu et al. tested the nanoelectrodes after EBID by Raman, and the Raman spectra showed that the deposit was sp2-rich amorphous carbon, which offered the nanoelectrodes a π-conjugated surface. By using these nanoelectrodes, Prof. Liu’s group produced molecular devices by using DNA molecules. “Since the DNA has a strong π–π interaction with π-conjugated surface, the DNA molecules will assemble between the nanoelectrodes. And after assembly, typical I–V curves of DNA molecules are observed, which means that these nanoelectrodes are available for the use in molecular devices.” Prof. Liu says, “in previous research, DNA molecules have been connected in circuit by Au nanoelectrodes or scanning probe microscope tips, and the current flows through the electrode/DNA interface by tunneling barriers or chemical bonds. However, in our case, the current through the interface by the π–π stacking between the nanoelectrodes with π-conjugated surfaces and the DNA molecules, thus the π–π stacking can also provide a well contact” (Fig. ​(Fig.33). Figure 3 The electrical properties of a DNA device fabricated by using the CNT electrodes. The I–V curves are measured before (red) and after (black) the assembly of DNA on the nanoelectrode. The upper inset shows a scheme of the device, and the lower ... Prof. Liu’s group has contributed to the current state of molecular electronics by providing a simple and efficient method to fabricate nanoelectrodes with controlled gap size with a real time and in situ characterization. It will be most valuable for the current efforts to investigate or realize molecular electronics and nanoelectronics.

Suppressing recombination processes and improving the minority carrier lifetime are critical for enhancing the performance of solar cells. Analysis of the balance between generation, recombination, and transport and its effect on the photocurrent and open circuit voltage of solar cells as well as a review of modern approaches employed to overcome the Shockley–Queisser limit are presented.

Optoelectronic materials for advanced IR sensing should combine wide strong electron coupling to the IR radiation, spectral tunability, adjustable dynamic range, manageable trade-off parameters, such as the noise characteristics and the operating time. Modern nanomaterials based on quantum dots and quantum wells provide wide possibilities to manage photoelectron processes via tuning the charge of quantum dots and quantum wells by the electric field and/or optical pumping. Variations in charge built in dots and wells change spectral characteristics, photocarrier lifetimes, and noise processes. These effects are especially strong in nanomaterials with strong selective doping of dots and wells. Manageable built-in charge provides wide possibilities to control the spectra, detector responsivity, and recombination processes.

Most of investigations of quantum dot photovoltaic devices are aimed at the development of the intermediate band solar cell. To form the intermediate band by quantum dot electron levels, the dots should be placed close to one to another. This leads to strain accumulation and defects, which increase the photocarrier recombination, and recombination losses. To avoid the nanostructuring-induced recombination, we proposed and studied an alternative approach, which is based on the separation of quantum dots (QDs) or QD clusters from the conducting channels by potential barriers created by quantum dots with built-in charge (Q-BIC). Charging of QDs improves the performance of QD solar cells due to the following factors: Negative dot charging increases electron coupling to sub-bandgap photons and provides effective harvesting of IR energy. Because of the strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs act as deep traps for electrons, but they are shallow traps for holes. Thus, the holes trapped in QDs may be excited by thermal phonons, while excitation of localized QDs electrons requires IR radiation or the interaction with hot electrons. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Charging of QDs is also an effective tool for managing the potential profile at micro- and nanoscales. Filling QDs predominantly from dopants in the QD medium allows one to maintain the macroscale profile analogous to that in the best conventional single-junction solar cells.

Novel approach to control of photoelectron processes is based on nanoscale engineering of 3D potential profile employing quantum dots with built-in charge (Q-BIC). Charging of quantum dots creates local potential barriers around single dots, if dots are arbitrary distributed in the medium, and collective barriers around dot clusters, rows etc, if quantum dots form specific structures. Manipulations with potential barriers provide an effective tool for suppression of fast capture processes of photocarriers by quantum dots. This allows one to increase the photocarrier lifetime and to reduce the recombination losses. The charging of dots also enhances the electron coupling to infrared radiation and multi-step absorption of sub-gap photons. Q-BIC nanomaterials have a number of attractive features to be used in photovoltaic and sensing applications.

Thermoelectric and solar power generation are two promising alternative energy solutions for the military. Power generation devices based on these two technologies aid the military by lightening Soldiers? loads and allowing them to carry fewer batteries and more equipment. Since these technologies use renewable energy sources, Soldiers are also spared the nuisance of having to constantly resupply. Thermoelectric power generates electricity from any adventitious source of heat, while solar cells employ sunlight to do the same. Both technologies are currently subject to extensive research to improve efficiency, utility, and cost. Although they both operate on renewable energy sources, each technology functions at a different temperature range. Thermoelectric devices run on low temperatures, generally below 1000 K, while solar cells require temperatures equivalent to those of the sun. Thermoelectric and solar power generators complement each other since they are capable of producing electricity from disparate sources. Solar produces highly efficient power during daytime but fails in darkness. Thermoelectrics can produce electricity with lower efficiency but dramatically more sources of heat are available for thermoelectric power generation, enabling full diurnal operation. This report discusses measurements of the properties and efficiency of a thermoelectric device and a solar cell.

We present theoretical and experimental results of electron kinetics and transport in quantum dot structures with potential barriers created around dots via intentional or unintentional doping. Monte Carlo simulations demonstrate that photoelectron capture is substantially enhanced in strong fields and electron kinetics can be controlled by potential barriers. Therefore, by creating potential barriers around dots, we found that our novel quantum dots with built-in charge (Q-BIC) solar cells and infrared (IR) photodectors enhance electron intersubband transitions and suppress fast electron capture processes. These factors lead to a 60% increase in the photocurrent of the Q-BIC solar cells (without degradation of the open circuit voltage) and ~25 times increase in the photoresponse of the Q-BIC photodetectors.

We investigated the effects of doping on the photovoltaic efficiency in a GaAs reference cell, and in undoped, n-doped, and p-doped InAs/GaAs quantum-dot (QD) solar cells. We found that the photovoltaic efficiency of the undoped QD solar cell is almost the same as that of the reference cell. However, the efficiency improves monotonically with increasing inter-dot ndoping, while p-doping deteriorates the photovoltaic conversion. We observed a 50 % increase in photovoltaic efficiency in the device n-doped to provide approximately six electrons per dot as compared with the undoped QD cell. In this QD solar cell, the short circuit current density increases to 24.30 mA/cm 2 compared with 15.07 mA/cm 2 in the undoped QD solar cell without deterioration of the open circuit voltage. To identify the physical mechanisms that provide this improvement, we investigated the spectral characteristics of the photovoltaic response and photoluminescence of our QD solar cells. We found that the electron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The electrons trapped in dots enhance IR transitions. The built-in-dot electron charge together with charged dopants outside the dots creates potential barriers, which suppress the fast electron capture processes and at the larger scale form a potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and a radical improvement of the QD solar cell efficiency. Higher efficiencies are anticipated with further increase of doping level and at higher radiation intensity. This makes the QD solar cells promising candidates for use with concentrators of solar radiation.

We present a comparative study for the evolution of utilizing indium gallium (InGa) and aluminum gallium (AlGa) alloys fabricated on GaAs(100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lack the ability to precisely determine the final alloy composition as well as consistency in the density of the droplets. Further, the composition of the InGa alloy is not uniform, as seen by the size distribution using an atomic force microscope (AFM). Although this approach may be acceptable for materials with similar surface kinetics, as in the case of AlGa, it is not acceptable for InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition for plasmonic applications such as plasmonic waveguides.

We report a 50% increase in power efficiency for our novel InAs/GaAs quantum dot with built-in charge (Q-BIC) solar cells. We found that n-doping the inter-dot space of a quantum dot solar cell (QDoSC) increases the short circuit current density from 15.07 mA/cm2 in undoped QDoSC to 24.30 mA/cm2 in the device doped to provide approximately six electrons per dot. To identify the physical mechanisms that provide this significant improvement, we investigate the photovoltaic response and its spectral characteristics in GaAs reference cell, undoped, n-doped, and p-doped QDoSCs. We found that the photovoltaic efficiency of the undoped QDoSC is almost the same as that of the reference cell. The efficiency monotonically improves with increasing n-doping, while the p-doping deteriorates the photovoltaic conversion. Studies of the photoluminescence of p- and n-doping show that the photoelectron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The built-in-dot electron charge enhances electron inter-subband QD transitions, suppresses the fast electron capture processes, and together with charged donors, forms the potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and to a radical improvement of the QDoSC efficiency. Even higher efficiencies are anticipated for higher n-doping levels.

Semiconductor quantum dots have long been described as artificial atoms because they have discrete energy states analogous to the energy levels of natural atoms. In recent years, it has become possible to create coupled pairs of quantum dots that are analogous to natural diatomic molecules. These artificial molecules have received a great deal of attention because of the potential applications in novel optoelectronic and spintronic devices, including the possibility of scalable implementations of quantum information processing. Just as in natural diatomic molecules, tunneling of electrons or holes between the two dots creates delocalized molecular orbitals. In natural diatomic molecules the molecular ground state has bonding orbital character and the first excited molecular state has antibonding character. Artificial quantum dot molecules were believed to behave in a similar way. However, recently Dr. Doty from the University of Delaware, Dr. Climente from Universitat Jaume I (Castellon, Spain), and their collaborators in Canada and the US, have experimentally verified and explained the existence of an antibonding molecular ground state for holes in artificial quantum dot molecules. The coherent coupling of quantum dots leads to the formation of delocalized molecular orbitals that appear in photoluminescence spectra under electric fields as ant crossings. The orbital character of the molecular states cannot be measured at zero magnetic field. However, a recent discovery by Doty and coworkers at the Naval Research Lab revealed that, when a magnetic field was applied, the resonant changes in the Zeeman splitting that depended on the orbital character of the molecular states appeared. Using these changes to identify the molecular orbital character, Doty and coworkers demonstrated that the orbital character of the molecular ground state reverses as a function of the distance between the dots. Holes confined in the molecular states of dots separated by 2 nm have the expected bonding molecular ground state; when the dots are separated by three or more nanometers, the molecular ground state becomes antibonding. “This was a surprising discovery for us,” said Doty. “Until we looked closely at the orbital character of the molecular states we had simply assumed that the molecular ground state was always a bonding orbital.” To explain this surprising result, Climente and coworkers utilized a four-bandk · papproximation which shows that the parity along the molecular axis is broken by the spin-orbit interaction in the valence band. “This leads to the mixing of bonding and antibonding heavy- and light-hole components of the spinor which destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal” Climente told Nanospotlight. He continued explaining that the molecular ground states are found to have up to ~95% antibonding character as a result of this mixing, “This is about 10 times higher than the largest value observed in natural molecules, so we can speak about a novel kind of molecular state whose properties are still to be explored”. Asp3d5s* tight binding multimillion atom calculation was applied to probe theoretically a real case. “We considered self-assembled InGaAs/GaAs double quantum dot structures and included strain, structural asymmetries as well as vertical electric fields. The results are in qualitative agreement with thek · ptheory and predict a bonding-to-antibonding ground state reversal at interdot distances ofd ~ 2 nm” says Climente. This discovery drastically changed the previous conception of the single-particle ground state of holes in coupled quantum dots. “This paves the way for more accurate simulations of device performance for applications in optics, transport, or quantum information” Climente mentioned. He continued saying that the tunneling rate, which is an important parameter for the quantum computation and transport can now be flexibly tuned from large values down to zero using a magnetic field. “This is a consequence of the spinor nature of holes” says Climente, “and it shows that we can greatly manipulate the properties of artificial molecules through the spin-orbit interaction”. As a matter of fact, this discovery provides new tools for engineering the spatial distribution of molecular wavefunctions in regions with varying material parameters. “Wavefunction engineering can be used to control magnetic, spin, and optical properties, so this discovery will enable the design of wide variety of materials for novel optoelectronic applications,” said Doty (Fig. ​(Fig.11). Figure 1 Dissociation spectrum of a hole in an artificial molecule (solid line). Note the state reversal atd ~ 1.5 nm, implying a bonding-to-antibonding ground state transition. This never occurs for electrons in artificial or natural diatomic molecules, where ... This work is also featured in the OAtube nanotechnology journal,http://www.oatube.org/2008/09/jiclimente.html. This journal is a new kind of open access effort that offers video access to science.

Although nanowires have attracted much interest in sensing applications, polymer single nanowires for optical sensing, which promises greater versatility and superior performances, remains unexplored. To date, most of these single nanowire devices have been focused on the electrical conductance change when exposed to the species. However, optical detection are highly desired owing to its advantages, such as fast response, immunity to electromagnetic interference, safe operation in explosive or combustive atmosphere, the possibility of remote monitoring through optical fibers. Researchers in Zhejiang University, China, reported a novel approach to polymer single-nanowire optical sensors. The compact and flexible sensing scheme demonstrated may be attractive for very fast detection in physical, chemical, and biological applications with high sensitivity and small footprint. Fuxing Gu and Professor Limin Tong who leads the Nanophotonics group at the Zhejiang University in China, together with coauthors from Department of Chemistry at Zhejiang University, report their most recent findings in an article titled “Polymer Single-Nanowire Optical Sensors” published online on August 2, 2008 in the journalNano Letters. “When a weak stream of light is guided along a properly functionalized polymer nanowire, the light output is strongly and instantly dependent on the surrounding of the nanowire, making it ideal for high-sensitivity sensing with ultrafast response.” Prof. Tong explained the basic idea. “Polymer single nanowires for gas optical sensing are of great scientific interest and technological significance” Gu explains to Nanospotlight. “To use polymer single nanowires as optical gas sensors, two major challenges must be overcome: the fabrication of polymer single nanowires that are optically sensitive to specific specimens, and efficiently launching light into and picking signals up from the nanowire. First, we draw nanowires from polymer solutions doped or blended with functional materials that act as sensitive elements. Secondly, we employ an evanescent coupling technique for high-efficient optical launching and collection.” “When we use a polyacrylamide (PAM) nanowire (Fig. ​(Fig.1)1) for relative humidity (RH) sensing, we got a response time as short as 30 ms, which are over 10 times faster than those of existing RH sensors” says Gu. Polyaniline blended with polystyrene was used to detect NO2at concentrations as low as 0.1 ppm with fast response, good reversibility, and fast recovery time at room temperature. Bromothymol blue (a pH indicator)-doped poly(methyl methacrylate) nanowires were used to detect NH3down to 3 ppm level with fast response. Figure 1 PAM single-nanowire humidity sensor—Schematic illustration of the sensor. Inset, optical microscope image of a MgF2-supported 410-nm-diameter PAM nanowire with a 532-nm-wavelength light launched from left side. Thewhite arrowindicates the direction ... “The remarkably fast response of the sensor can be attributed to the small diameter and large surface-to-volume ratio of the nanowire that enable rapid diffusion or evaporation of the water molecules, as well as fast signal collection using optical approach.” Gu explains. The seamless combination of the functionalized polymer single nanowire with optical scheme demonstrated here presents high versatility and flexibility for gas sensing, and may open new opportunities for both polymer nanowires and optical sensors. “The optical scheme extends the sensing element from conductive polymer nanowires for electric detection to broader categories, since a majority of polymer nanowires, no matter they are electric conductive or not, provide low optical waveguiding losses (Fig. ​(Fig.2)2) within visible or near infrared spectral range. Moreover, compared with other materials, polymers are much more hospitable to a variety of dopants.” says Prof. Tong. “We are now working to use polymer single nanowires for biological sensing integrated with microchip, and investigating optoelectronics and nonlinear effects in polymer single nanowires for nanoscale devices.” Figure 2 Waveguiding polymer single nanowire—optical micrographs of a 300-nm-diameter PMMA nanowire guiding a broadband supercontinuum (denoted as SC) and monochromatic lasers with wavelengths of 488, 532, 660, and 980 nm, respectively. Scale bar, 50 μm. ...

Nanohole formation on an AlAs/GaAs superlattice gives insight to both the “drilling” effect of Ga droplets on AlAs as compared to GaAs and the hole-filling process. The shape and depth of the nanoholes formed on GaAs (100) substrates has been studied by the cross-section transmission electron microscopy. The Ga droplets “drill” through the AlAs layer at a much slower rate than through GaAs due to differences in activation energy. Refill of the nanohole results in elongated GaAs mounds along the [01−1] direction. As a result of capillarity-induced diffusion, GaAs favors growth inside the nanoholes, which provides the possibility to fabricate GaAs and AlAs nanostructures.

Recently, droplet heteroepitaxy of GaAs/AlGaAs has attracted much attention due to its demonstrated capability of producing nanostructures with interesting geometrical shapes such as quantum single and double ringlike structures. In the droplet-heteroepitaxy approach, Ga is deposited on an AlGaAs surface to create liquid Ga droplets, and these droplets are exposed to an As flux that transforms the droplets into GaAs nanocrystals. In principle, this same approach can be modified into a similar process of GaAs homoACHTUNGTRENNUNGepitaxy, which also allows one to structure GaAs surfaces at the nanometer scale. In this droplet-homoepitaxy approach, the nanostructures formed by the droplets can then be used to encourage the growth of subsequent heteronanostructures such as self-assembled InAs quantum dots (QDs). In this self-assembly process, the lattice mismatch between InAs and GaAs encourages the spontaneous formation of QDs once InAs deposition reaches a critical thickness. These self-assembled InAs QDs have been extensively investigated for the past decade because of their applications in advanced devices such as lasers, infrared detectors, and quantum computers. However, the stochastic nature of self-assembly only permits a random distribution of InAs QDs on GaAs surfaces, which presents a major obstacle for applications such as quantum computing. Recently, much effort has been put forth to localize QDs on designed regions of substrates. By combining the self-assembly approach with certain lithography techniques, InAs QDs can be restricted to growth in specific substrate regions due to GaAs surface height modulations induced by the lithography. In this paper, however, we propose and demonstrate an approach using only self-organizing techACHTUNGTRENNUNGniques to localize the QDs, a hybrid process of GaAs droplet epitaxy and strain-driven self-assembly of InAs QDs. By using droplet epitaxy, GaAs ACHTUNGTRENNUNG(100) surfaces are structured with nanoscale mounds elongated along the [011] direction. The GaAs mounds then induce localized formations of InAs QDs, and these quantum-dot clusters (QDCs) are observed around the GaAs mounds. The density of QDCs and the number of QDs in the clusters can be adjusted by controlling the growth conditions of the Ga droplets. Photoluminescence (PL) measurements reveal atypically good optical properties of these QDCs. The samples were grown by molecular-beam epitaxy (MBE) on epitaxy-ready GaAsACHTUNGTRENNUNG(100) substrates. All growth began with a 300-nm GaAs buffer layer, which was followed by 10 monolayers (ML) of Ga deposition to form the liquid droplets. Subsequently, the Ga droplets were exposed to an As molecular flux and annealed for 2 min at the substrate temperature of 530 8C to fully crystallize the droplets into GaAs surface nanostructures. At the same substrate temperature of 530 8C, InAs was then deposited until the QDs formed after reaching a critical coverage of 1.7 ML. The resulting surface morphologies were imaged by atomic force microscopy (AFM) under ambient conditions. For PL meaACHTUNGTRENNUNGsurements, the resulting InAs QDs were capped with 100 nm of GaAs. Each PL measurement was performed at a temperature of 10 K by using 532-nm laser excitation. While Ga deposition with a concurrent As molecular flux encourages two-dimensional homoepitaxial growth on GaAs surfaces, Ga deposition without the concurrent As flux results in the formation of liquid droplets, as schematically represented in Figure 1a. After these droplets subsequently react with the As, they are transformed into GaAs nanostructures, normally taking an anisotropic shape due to the dropletsA orientation-dependent surface diffusion on the GaAs crystal surfaces. Figure 1b shows a schematic repre

Effects of concentrated solar radiation on photovoltaic performance are investigated in well-developed GaAs quantum dot (QD) solar cells with 1-Sun efficiencies of 18%–19%. In these devices, the conversion processes are enhanced by nanoscale potential barriers and/or AlGaAs atomically thin barriers around QDs, which prevent photoelectron capture to QDs. Under concentrated radiation, the short circuit current increases proportionally to the concentration and the open circuit voltage shows the logarithmic increase. In the range up to hundred Suns, the contributions of QDs to the photocurrent are proportional to the light concentration. The ideality factors of 1.1–1.3 found from the VOC-Sun characteristics demonstrate effective suppression of recombination processes in barrier-separated QDs. The conversion efficiency shows the wide maximum in the range of 40–90 Suns and reaches 21.6%. Detailed analysis of I-V-Sun characteristics shows that at low intensities, the series resistance decreases inversely proport...

Incorporation of quantum dots into the p-n junction of the photovoltaic device provides numerous possibilities for nanoscale control of photoelectron processes via engineering the band structure and nanoscale potential profile. The band structure is determined by the size and shape of quantum dots and width of the wetting layer. The potential profile is formed by selective doping, which leads to the built-in dot charge. To study the effects of the band structure and nanoscale potential barriers on the photovoltaic performance we fabricated and investigated 3-μm base GaAs devices with various InAs quantum dot media and selective doping. All quantum dot devices show significant improvement in conversion efficiency in comparison with the reference cell. The data obtained have been analyzed in frame of the diode model. It was found that the two-diode model with the ideality factors of [Formula: see text] and [Formula: see text] well describes the scope of data. The essential [Formula: see text] component evidences that the Shockley-Read-Hall recombination plays a substantial role in recombination losses. Further optimization of solar cells should be aimed at the formation of potential profile with large barriers that separate quantum dot areas from high mobility conducting channels.

Self-assembled InAs quantum dot clusters (QDCs) and InGaAs QD molecules (QDMs) have been demonstrated through a growth technique called “droplet epitaxy” by molecular beam epitaxy (MBE). For QDCs, the size and density of QDs can be controlled with variation of InAs monolayer coverages. For QDMs, Ga contribution from GaAs mound with the interaction of InAs deposition resulted in various number of InGaAs QDs per GaAs mound, ranging from 2 to 6 (bi-QDMs to hexa-QDMs) depending on the specific InAs monolayer deposition. High step density on sidewall of GaAs mound and anisotropy of surface diffusion gave a rise to preferential formation of InAs and InGaAs QDs around GaAs mounds. This hybrid growth approach combining droplet epitaxy and typical QD growth is relatively simple and flexible and doesn't require further ex-situ surface preparation. This approach of QD arrangement can find applications in optoelectronics as well as physical study of QD interaction.