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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.

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.

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.

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 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)

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.

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.

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 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.

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. ...

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...

Quantum dots (QDs) provide photovoltaic conversion of below-bandgap photons due to multistep electron transitions. QDs also increase conversion efficiency of the above-bandgap photons due to extraction of electrons from QDs via Coulomb interaction with hot electrons excited by high-energy photons. Nanoscale potential profile (potential barriers) and nanoscale band engineering (AlGaAs atomically thin barriers) allow for suppression of photoelectron capture to QDs. To study these kinetic effects and to distinguish them from the absorption enhancement due to light scattering on QDs, we investigate long, 3-μm base GaAs devices with various InAs QD media with 20 and 40 QD layers. Quantum efficiency measurements show that, at least at low doping, the multistep processes in QD media are strongly affected by the wetting layer (WL). The QD media with WLs provide substantial conversion of below-bandgap photons and for devices with 40 QD layers the short circuit current reaches 29.2 mA/cm2. The QD media with band-engineered AlGaAs barriers and reduced wetting layers (RWL) enhance conversion of high-energy photons and decrease the relaxation (thermal) losses.

Single molecule labeling, cancer treatment, enhancement of non-linear optical effects, or light guiding have demanded much attention from the scientific community, and as a possible solution, plasmon resonances of noble metal nanoparticles are explored. A major advancement in single molecule optics has been the polarization analysis of light from single fluorescent emitters. This analytical method has been utilized to study the conformational dynamics of biomolecules and their spatial arrangement. At different wavelengths of the excitation light, different oscillation modes are excited making it important to know the polarization pattern as a function of wavelength. “Knowing the polarization pattern of plasmonic nanostructures is therefore not only important to understand the fundamental physics of light interaction with these structures, but also allows to discriminate different oscillation modes within one particle and to distinguish differently shaped particles within one sample. Several techniques have been used to extract optical spectra of single plasmonic nanoparticles, most efficiently using dark-field microscopy, but little is known about the polarization state. So far, the very few reported plasmon polarization studies were obtained by rotating a polarizer by hand or on ensembles and not combined with spectroscopic information,” Prof. Carsten Sonnichsen explains to Nano Spotlight. “We have developed a new microscope setup (RotPOL), which allows obtaining polarization-dependent scattering spectra in a fast and easy way,” Olaf Schubert continues explaining to Nano Spotlight. “RotPOL uses a wedge shaped quickly rotating polarizer which splits the light of a point source into a ring in the image plane, encoding the polarization information in a spatial image.” (Scheme ​(Scheme11) Scheme 1 (a) Schematics of the RotPOL setup. One wavelength is selected by a linear variable interference filter (varIF), and then the light is dispersed into different polarization directions by a wedge-shaped rotating polarizer (PL), resulting in ring-shaped intensity profiles of a point-like light source on the digital camera (b). In order to get the polarization profile shown in (c) (intensityI(q) as a function of polarization angle q), we integrate the image between an inner and an outer ring diameter (dashed lines in (b)). The center of the rings is chosen to minimize asymmetry between opposite sides. Repeating this procedure for each wavelength produces intensity values as a function of wavelength and polarization angleI(l,q), which we show color-coded in (d). The same analysis is possible for all particles within the field of view in parallel. (e) Real-color image of an inhomogeneous silver sample containing spheres, rods, and triangles as seen through the RotPol-microscope. Two colors in one ring correspond to two different plasmon modes at the respective wavelengths. Scalebar is 25 μm Prof. Sonnichsen’s team reveals that the polarization intensity in a given direction is simply taken from the corresponding position on the ring recorded with an exposure time larger than the rotation time. A dipole, for example, will show two loops at opposite sides, and with this in mind, the team can combine this rotating polarizer with a variable wavelength interference filter, which transmits light only in a narrow wavelength window. If mounted in front of the digital camera, the filter allows them to record simultaneously the spectral and polarization information for up to 50 particles in parallel. “With the RotPOL setup, we study plasmon modes of a large variety of plasmonic structures—from rod-shaped particles to triangles, cubes, and pairs of spheres,” said Olaf Schubert. “Each plasmonic particle has a characteristic ‘footprint’, which allows deducing the approximate particle shape from the polarization-dependent single-particle scattering spectra. This is important for the optimization of particle synthesis, because it makes a quick and efficient estimation of the quality and mono-dispersal of a sample possible, without complex and expensive tools like electron microscopy.” “For rod-shaped and even just slightly elongated particles, we found that the scattered light is highly polarized. Our simulations show that this high polarization anisotropy is not only due to the particle symmetry, but a plasmonic effect. This could be exploited for the design of miniature rotation sensors,” Prof. Sonnichsen explained enthusiastically. In addition to yielding orientation information, plasmonic particles can be used to measure absolute distances on a nanometer scale. “Such a ‘plasmonic ruler’ makes use of the coupling of two spherical particles: If they are close to each other, the inter-particle plasmon resonance shifts from green to red. We measured the full polarization-dependent spectrum of such pairs of two spheres and found nice agreement with simulations.” This investigation has demonstrated that the polarization-dependent spectrum contains information about both the distances of the two spheres and the orientation and environment of the particles. In addition, such “multi-sensors” could possibly find a place in biological applications that require high time resolution. “As an example of a time-resolved measurement, we have monitored the changes of plasmon modes in single gold nanoparticles during a growth process, in situ,” explains Olaf Schubert to Nano Spotlight. The researchers highlight that their RotPOL method is a versatile tool that can be used to study polarization anisotropy of the light emission pattern from nanoparticles, particularly for plasmonic structures, but can possibly be extended to fluorescent quantum structures. This method provides a wide range for optimization for applications such as light guiding and allows detailed theoretical modeling of plasmon modes due to the wide variety of plasmon emission patterns observed for the simple particle morphologies that have been investigated (spheres, rods, triangles, cubes, and particle pairs). The researchers have recently published their results in Nano Lett, 2008. Their work reveals that the high polarization anisotropy found for even moderately elongated spheres “highlights” the strong influence of polarization even for nominally round particles. The possibility to record dynamic changes of the polarization emission pattern of single particles allows studying particle growth modes in situ and improving schemes for single nanoparticle binding and distancing assays.

Quantum dots acquire built-in charge due to selective n-doping of the interdot space. The quantum dots with built-in charge (Q-BIC) increase electron coupling to IR radiation and suppress photoelectron capture, which in turn decrease the recombination via quantum dots. To investigate effects of the built-in-dot charge on recombination processes and device performance, the light and dark I–V characteristics and their temperature dependences of Q-BIC solar cells are measured. Employing the diode model, the data are analyzed in terms of the ideality factor, shunt resistance, and reverse saturation current. The authors compare the n-doped Q-BIC solar cells with the GaAs p-i-n reference cell, undoped, and p-doped devices. The analysis provides a qualitative description of the effect of doping on carrier kinetics and transport. The authors show that n-doping substantially reduces the recombination via quantum dots.

We have studied the effects of AlGaAs energy barriers surrounding self-assembled InAs quantum dots in a GaAs matrix on the properties of solar cells made with multiple quantum dot layers in the active region of a photodiode. We have compared the fenced dot samples with conventional InAs/GaAs quantum dot solar cells and with GaAs reference cells. We have found that, contrary to theoretical predictions, the AlGaAs fence layers do not enhance the transport properties of photogenerated carriers but instead suppress the extraction of the carriers excited in the dots by light with wavelengths longer than the cutoff wavelength of the GaAs matrix material. Both the standard quantum dots and the fenced dots were found to give solar cell performance comparable to the GaAs reference cells for certain active region thicknesses but neither showed enhancement due to the longer wavelength absorption or improved carrier transport.

Uses of semiconductor quantum dots (QDs) have been explored for attractive applications such as quantum information processing, lasers and solar cells to name a few. However, their ability to become integrated with existing technologies depends on shape, composition and size, all of which play a role in modifying both the optical and electrical properties of the QDs. However, an in-depth understanding of the structure-property relationship of QDs is still lacking. In an effort to expand the understanding of this relationship, scientists from the Radboud University Nijmegen (The Netherlands), Eindhoven University of Technology (The Netherlands) and the Ruhr-Universitat in Bochum (Germany) have employed the use of crosssectional scanning tunneling microscopy (X-STM), a technique perfected by Paul Koenraad’s group at the COBRA Inter-University Research Institute in Eindhoven, to investigate and determine the structural composition of InAs QDs. The results of the work entitled, ‘‘Ellipsoidal InAs quantum dots observed by cross-sectional scanning tunneling microscopy’’ was published in Applied Physics Letters. ‘‘We were surprised to find that the shape and composition deviate strongly from those which are usually used’’, says Janneke Blokland, PhD student at the Nijmegen High Field Magnet Laboratory (HFML) and first author of the paper. ‘‘The dots have an ellipsoidal shape and turned out to be much taller and with a stronger indium gradient than expected’’. The report was very interesting since both the optical and electrical properties of the ellipsoidal QDs were earlier determined and reported in a Physical Review B paper ‘‘Hole levels in InAs self-assembled quantum dots’’. ‘‘This is the first time that structural and spectroscopic properties of quantum dots are experimentally combined’’, says Peter Christianen, associate professor at the HFML Nijmegen. According to Christianen, spectroscopic data was only recently explained by theoretical models with the use of the QD dimension and composition as fitting parameters. ‘‘These simple models resulted in dot parameters that are really different than the ones we have found now’’ explains Christianen to Nanospotlight. ‘‘With our unique combination of spectroscopy and X-STM experiments on the same QDs, we make it possible to really test advanced theoretical models to understand the underlying physics at the nanoscale and use this knowledge to improve the performance of future devices’’.