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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 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%.

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

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.

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

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.

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.

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.

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.

While the properties of individual quantum dots (QDs) are deemed attractive for applications in quantum computing, practically, they are insufficient. However, being able to group QDs to enable communication among them (quantum dot molecules—QDMs) is still extremely challenging although this field has been studied for about 20 years. Therefore, in an attempt to control the lateral ordering of QDs, researchers at the University of Arkansas have developed a simple hybrid technique to fabricate QD pairs (QDPs) and QDMs. This technique has shown promise and paves the way for investigating possible interactions between QDs, taking us one step further to realizing quantum computing.

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.

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’’.