PbS Quantum Dots

We report the synthesis and characterization of Carbon and CdTe quantum dots (QDs), as well as the observed improvement in the power conversion efficiency (PCE) of photovoltaic devices upon the incorporation of the synthesized aforementioned nanostructures. Even though C quantum dots were observed to have a relatively smaller influence on solar cell performance, they are considered to be a more attractive option due to their affordability and minimal impact in the environment that could ultimately promote their widespread utilization on photovoltaic structures.

In the last two decades, many experiments were conducted in self-organization of nanocrystals into two-and three-dimensional (3D) superlattices and the superlattices were synthesized and characterized by different techniques, revealing their unusual properties. Among all characterization techniques, X-ray diffraction (XRD) is the one that has allowed the confirmation of the 3D superlattice formation due to the presence of sharp and intense diffraction peaks. In this work, we study self-organized superlattices of quantum dots of PbS prepared by dropping a monodispersed colloidal solution on a glass substrate at different temperatures. We showed that the intensity of the low-angle XRD peaks depends strongly on the drying time (substrate temperature). We claim that the peaks are originated from the 3D superlattice. Scanning electron microscopy images show that this 3D superlattice (PbS quantum dots) is formed in flake's shape, parallel to the substrate surface and randomly oriented in the perpendicular planes.

Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics and other important optoelectronic properties. We capture such nanometer resolution images of electronically excited PbS quantum dots by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ~1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the quantum dots under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the quantum dots. Excitation of ad...

A simple optical model is presented to describe the influence of a planar luminescent down-shifting layer (LDSL) on the external quantum efficiencies of photovoltaic solar cells. By employing various visible light-emitting LDSLs based on CdTe quantum dots or CdSe/CdS core−shell quantum dots and tetrapods, we show enhancement in the quantum efficiencies of thin-film CdTe/CdS solar cells predominantly in the ultraviolet regime, the extent of which depends on the photoluminescence quantum yield (PLQY) of the quantum dots. Similarly, a broad enhancement in the quantum efficiencies of crystalline Si solar cells, from ultraviolet to visible regime, can be expected for an infrared emitting LDSL based on PbS quantum dots. A PLQY of 80% or higher is generally required to achieve a maximum possible short-circuit current increase of 16 and 50% for the CdTe/CdS and crystalline Si solar cells, respectively. As also demonstrated in this work, the model can be conveniently extended to incorporate LDSLs based on organic dyes or upconverting materials. Article pubs.acs.org/JPCC

In the present work a simple chemical reduction route has been followed to grow CdSe (size controlled) nanoparticles at room temperature. The grown sample has been ultrasonicated in ethanol. The dispersed sample has been characterized structurally, optically, and electrically. The result supports the formation of nanoparticles and hence an increase in band gap compared to bulk CdSe (bulk band gap is 1.74 eV).

An optical switch with two distinct resonances is formed by combining PbS nanocrystals and the conductive polymer poly[sodium 2‐(2‐ethynyl‐4‐methoxyphenoxy)acetate] (PAE) into a hybrid thin film. Infrared excitation of the nanocrystals invokes charge transfer and consecutive polaron formation in the PAE, which activates the switch for excited‐state absorption at visible frequencies. The optical modulation of the photocurrent response of the switch exhibits highly wavelength‐selective ON/OFF ratios. Transient absorption spectroscopy shows that the polaron formation is correlated with the excited state of the nanocrystals, opening up new perspectives for photonic data processing. Such correlated activated absorption can be exploited to enhance the sensitivity for one optical signal by a second light source of different frequency as part of an optical amplifier or a device with AND logic.

We have studied the optical absorptance of lead sulfide (PbS) quantum dot (QD) coated thin crystalline layered material (TCLM) experimentally and numerically. Starting with the synthesis, fabrication and characterizations, a sample of PbS QDs deposited on trilayer molybdenum disulphide (MoS 2) thin film has been probed locally using a reflection spectrometer setup. Since transmittance is needed to calculate the absorbtance of the QD film/TCLM sample, we run a set of simulations using a 3D finite-difference time-domain full-wave electromagnetic solver. Based on the agreement between experimental and numerical results for the reflectance spectra, which verifies the accuracy of our QD and TCLM modelling, we have calculated the absorptance. Unlike metal nanoparticle decorated TCLMs in which metal nanoparticles act like induced dipoles and enhance the absorptance, here we have not observed the similar effect; rather we have found that the absorptance of QD film/TCLM sample is almost equal to the summation of QDs" and TCLM"s individual absorptance in the wavelength range of 450-800 nm.

In this work we explore the preparation of complex-shaped semiconductor nanostructures composed of different materials via a cationic exchange process in which the cations of the original semiconductor nanostructure are replaced by cations of different metals with preservation of the shape and the anionic framework of the nanocrystals. Utilizing this cation exchange method, we synthesized two new tetrapods for the first time: Cu 2−x Se/Cu 2−x S and PbSe/PbS, both prepared from CdSe/CdS tetrapods as 'templates'. We also fabricated near-infrared (NIR) photodetectors with a very simple architecture comprising a PbSe/PbS tetrapod layer between two Au electrodes on a glass substrate. When illuminated by a NIR laser, these devices are capable of achieving a responsivity of 11.9 A W −1 without the use of ligand-exchange processes, thermal annealing or hybrid device architecture. Transient absorption spectroscopy was carried out on these PbSe/PbS tetrapods, the results of which suggest that the branched morphology contributes in part to device performance. Investigation of the charge dynamics of the PbSe/PbS tetrapods revealed an extremely long-lived exciton recombination lifetime of ∼17 ms, which can result in enhanced photoconductive gain. Overall, these heterostructured tetrapods showcase simultaneously the importance of nanoparticle shape, band structure, and surface chemistry in the attainment of NIR photodetection. † Electronic supplementary information (ESI) available. See

We have studied the optical absorptance of lead sulfide (PbS) quantum dot (QD) coated thin crystalline layered material (TCLM) experimentally and numerically. Starting with the synthesis, fabrication and characterizations, a sample of PbS QDs deposited on trilayer molybdenum disulphide (MoS 2) thin film has been probed locally using a reflection spectrometer setup. Since transmittance is needed to calculate the absorbtance of the QD film/TCLM sample, we run a set of simulations using a 3D finite-difference time-domain full-wave electromagnetic solver. Based on the agreement between experimental and numerical results for the reflectance spectra, which verifies the accuracy of our QD and TCLM modelling, we have calculated the absorptance. Unlike metal nanoparticle decorated TCLMs in which metal nanoparticles act like induced dipoles and enhance the absorptance, here we have not observed the similar effect; rather we have found that the absorptance of QD film/TCLM sample is almost equal to the summation of QDs" and TCLM"s individual absorptance in the wavelength range of 450-800 nm.

Temperature and Fermi energy dependent exciton eigenenergies of monolayer molybdenum disulfide (MoS 2) are calculated using an atomistic model. These exciton eigen-energies are used as the resonance frequencies of a hybrid Lorentz-Drude-Gaussian model, in which oscillation strengths and damping coefficients are obtained from the experimental results for the differential transmission and reflection spectra of monolayer MoS 2 coated quartz and silicon substrates, respectively. Numerical results compared to experimental results found in the literature reveal that the developed permittivity model can successfully represent the monolayer MoS 2 under different biasing conditions at different temperatures for the design and simulation of MoS 2 based opto-electronic devices.

Two dimensional superlattices of epitaxially connected quantum dots enable sizequantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we

In the last two decades, many experiments were conducted in self-organization of nanocrystals into two-and three-dimensional (3D) superlattices and the superlattices were synthesized and characterized by different techniques, revealing their unusual properties. Among all characterization techniques, X-ray diffraction (XRD) is the one that has allowed the confirmation of the 3D superlattice formation due to the presence of sharp and intense diffraction peaks. In this work, we study self-organized superlattices of quantum dots of PbS prepared by dropping a monodispersed colloidal solution on a glass substrate at different temperatures. We showed that the intensity of the low-angle XRD peaks depends strongly on the drying time (substrate temperature). We claim that the peaks are originated from the 3D superlattice. Scanning electron microscopy images show that this 3D superlattice (PbS quantum dots) is formed in flake's shape, parallel to the substrate surface and randomly oriented in the perpendicular planes.

Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics and other important optoelectronic properties. We capture such nanometer resolution images of electronically excited PbS quantum dots by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ~1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the quantum dots under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the quantum dots. Excitation of ad...

Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics and other important optoelectronic properties. We capture such nanometer resolution images of electronically excited PbS quantum dots by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ~1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the quantum dots under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the quantum dots. Excitation of ad...

Understanding the origins of the excessive Stokes shift in the lead chalcogenides family of colloidal quantum dots (CQDs) is of great importance at both the fundamental and applied levels; however, our current understanding is far from satisfactory. Here, utilizing a combination of ab initio computations and UV-vis and photoluminescence measurements, we investigated the contributions to the Stokes shift from polydispersity, ligands, and defects in PbS CQDs. The key results are as follows: (1) The size and energetic disorder of a polydisperse CQD film increase the Stokes shift by 20 to 50 meV compared to that of an isolated CQD; (2) Franck-Condon (FC) shifts increase as the electronegativities of the ligands increase, but the variations are small (<15 meV). (3) Unlike the aforementioned two minor factors, the presence of certain intrinsic defects such as V (in Cl-passivated CQDs) can cause substantial electron density localization of the band edge states and consequent large FC sh...

In the last two decades, many experiments were conducted in self-organization of nanocrystals into two-and three-dimensional (3D) superlattices and the superlattices were synthesized and characterized by different techniques, revealing their unusual properties. Among all characterization techniques, X-ray diffraction (XRD) is the one that has allowed the confirmation of the 3D superlattice formation due to the presence of sharp and intense diffraction peaks. In this work, we study self-organized superlattices of quantum dots of PbS prepared by dropping a monodispersed colloidal solution on a glass substrate at different temperatures. We showed that the intensity of the low-angle XRD peaks depends strongly on the drying time (substrate temperature). We claim that the peaks are originated from the 3D superlattice. Scanning electron microscopy images show that this 3D superlattice (PbS quantum dots) is formed in flake's shape, parallel to the substrate surface and randomly oriented in the perpendicular planes.

The electrostatic potential distribution across single, isolated, colloidal heterostructured nanorods (NRs) with component materials expected to form a p-n junction within each NR has been measured using scanning Kelvin probe microscopy (SKPM). We compare CdS to bicomponent CdS-CdSe, CdS-PbSe, and CdS-PbS NRs prepared via different synthetic approaches to corroborate the SKPM assignments. The CdS-PbS NRs show a sharp contrast in measured potential across the material interface. We find the measured built-in potential within an individual NR to be attenuated by long-range electrostatic forces between the sample substrate, cantilever, and the measuring tip. Surface potential images were deconvoluted to yield built-in potentials ranging from 375 to 510 meV in the heterostructured NRs. We deduce the overall built-in potential as well as the charge distribution across each segment of the heterostructured NRs by combining SKPM data with simulations of the system.

In this work we explore the preparation of complex-shaped semiconductor nanostructures composed of different materials via a cationic exchange process in which the cations of the original semiconductor nanostructure are replaced by cations of different metals with preservation of the shape and the anionic framework of the nanocrystals. Utilizing this cation exchange method, we synthesized two new tetrapods for the first time: Cu 2-x Se/Cu 2-x S and PbSe/PbS, both prepared from CdSe/CdS tetrapods as 'templates'. We also fabricated near-infrared (NIR) photodetectors with a very simple architecture comprising a PbSe/PbS tetrapod layer between two Au electrodes on a glass substrate. When illuminated by a NIR laser, these devices are capable of achieving a responsivity of 11.9 A W -1 without the use of ligand-exchange processes, thermal annealing or hybrid device architecture. Transient absorption spectroscopy was carried out on these PbSe/PbS tetrapods, the results of which suggest that the branched morphology contributes in part to device performance. Investigation of the charge dynamics of the PbSe/PbS tetrapods revealed an extremely long-lived exciton recombination lifetime of ∼17 ms, which can result in enhanced photoconductive gain. Overall, these heterostructured tetrapods showcase simultaneously the importance of nanoparticle shape, band structure, and surface chemistry in the attainment of NIR photodetection. † Electronic supplementary information (ESI) available. See

Molecular gating (via electric field from molecule's dipole-moment) is an effective route to dope two dimensional (2D) nanomaterials for specific electronic applications. Here, we show that molybdenum disulphide (MoS2) interfaced with a microfiber of hygroscopic polyelectrolyte can be reversibly doped with water molecules by changing the local humidity. In this work, the p-doping water absorbed by the polymer microfiber reduces the electron density in the n-type MoS2 quantum dots. This change in the carrier concentration (5.24 x 10 11 cm -2 ) is confirmed by electrical measurements. Further, Raman spectra have been analyzed to understand the bonding of molecular groups on MoS2 quantum dots and their self-assembly on the microfiber. The work provides an avenue to explore interfacing 2D quantum dots interfaced with other polymers for specific sensing applications.

An optical switch with two distinct resonances is formed by combining PbS nanocrystals and the conductive polymer poly[sodium 2-(2-ethynyl-4-methoxyphenoxy)acetate] (PAE) into ah ybrid thin film. Infrared excitation of the nanocrystals invokes charge transfer and consecutive polaron formation in the PAE, which activates the switch for excitedstate absorption at visible frequencies.The optical modulation of the photocurrent response of the switch exhibits highly wavelength-selective ON/OFF ratios.T ransient absorption spectroscopys hows that the polaron formation is correlated with the excited state of the nanocrystals,o pening up new perspectives for photonic data processing.S uch correlated activated absorption can be exploited to enhance the sensitivity for one optical signal by as econd light source of different frequency as part of an optical amplifier or adevice with AND logic.

We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material,. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping of the HgTe nanoplatelets by S 2ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 µs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100 ns to 500 ns range, which suggests that the current performances are rather limited by geometrical factors.

We propose a method for measuring the thickness of the exfoliated MoSe2 layers deposited on Si/SiO2 substrate, based on the reflectance measurements performed with laser light illumination at two different wavelengths: red and green from confocal microscope at room temperature. We demonstrate the correlation between the number of layers in a flake and the value of its relative reflection difference. We applied the transfer matrix method to calculate the reflectivity and verify our experimental results. The approach proposed by us allows for fast and automatic verification of the exfoliated MoSe2 layers thickness on large areas of the substrate.

We propose a method for measuring the thickness of the exfoliated MoSe2 layers deposited on Si/SiO2 substrate, based on the reflectance measurements performed with laser light illumination at two different wavelengths: red and green from confocal microscope at room temperature. We demonstrate the correlation between the number of layers in a flake and the value of its relative reflection difference. We applied the transfer matrix method to calculate the reflectivity and verify our experimental results. The approach proposed by us allows for fast and automatic verification of the exfoliated MoSe2 layers thickness on large areas of the substrate.

Rod-shaped CdTe-Cu 2Àx Te nano-heterostructures with tunable dimensions of both sub-units and a type II band alignment were prepared by Cd 2+ /Cu + cation exchange. The light absorption properties of the heterostructures are dominated by the excitonic and plasmonic contributions arising, respectively, from the CdTe and the Cu 2Àx Te sub-units. These results were confirmed over a wide range of sub-unit length fractions through optical modelling based on the discrete dipole approximation (DDA). Although assuming electronically independent sub-units, our modelling results indicate a negligible ground state interaction between the CdTe exciton and the Cu 2Àx Te plasmon. This lack of interaction may be due to the low spectral overlap between exciton and plasmon, but also to localization effects in the vacancydoped sub-unit. The electronic interaction between both sub-units was evaluated with pump-probe spectroscopy by assessing the relaxation dynamics of the excitonic transition. In particular, the CdTe exciton decays faster in the presence of the Cu 2Àx Te sub-unit, and the decay gets faster with increasing its length. This points towards an increased probability of Auger mediated recombination due to the high carrier density in the Cu 2Àx Te sub-unit. This indication is supported through length-fraction dependent band structure calculations, which indicate a significant leakage of the CdTe electron wavefunction into the Cu 2Àx Te sub-unit that increases along with the shortening of the CdTe sub-unit, thus enhancing the probability of Auger recombination. Therefore, for the application of type II chalcogenide-chalcogenide heterostructures based on Cu and Cd for photoenergy conversion, a shorter Cu-based sub-unit may be advantageous, and the suppression of high carrier density within this sub-unit is of high importance.

In this study, the influence of embedding cadmium sulfide nanoparticles, synthesised by a chemical method, on the performance of porous silicon matrix photodetectors was investigated. An anodization technique was used to fabricate porous silicon photodetectors at 16 mA/cm 2 for 10 min.The characteristics of porous silicon and CdS nanoparticles were investigated by using x-ray diffraction XRD, atomic force microscopy AFM, scanning electron microscopy SEM, and energy dispersive x-ray EDX. Dark and illuminated current-voltage I-V characteristics and spectral responsivity of photodetectors were investigated before and after the incorporation of CdS nanoparticles. Considerable improvement was noticed in responsivity of the porous silicon photodetector after incorporating the CdS nanoparticles into the silicon matrix.

Solution processed organic photodetectors with a nanostructured active layer in the shape of a photonic crystal exhibit an improved NIR response, below the band gap of the active layer materials, that can be tuned by varying the lattice parameter. 

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 (0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 (5% and 80 (7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.

The use of microorganisms for in vivo synthesis of quantum dots (QDs) not only makes it possible to obtain useful materials with tunable characteristics under mild conditions, but can also ensure the removal of toxic heavy metals from polluted water systems. Because real water treatment schemes contain more than one pollutant, this work attempted to determine the responses of Candida to exposure to two cations with known different survival pathways: PbIJII) and CdIJII). Five Candida species, C. albicans, C. dubliniensis, C. glabrata, C. krusei, and C. parapsilosis, were exposed to a 1 : 1 mixture of the aforementioned cations. The heterogeneous uptake of each metal was defined for the different strains; the removal of cadmium was greater than that of lead. The specific pathway followed by each cation led to the biomineralization of heterogeneous PbS- and CdS-based QDs located extra- or intra-cellularly. The structures and properties of the obtained heterogeneous nanomaterials were characterized by different techniques, and the results highlight the potential of Candida to form useful materials from a complex metal mixture.

Las aplicaciones de micro-y nano-cristales producidos por sistemas biológicos son de gran interés en diversas áreas tecnológicas y ambientales. La exposición de microorganismos a metales pesados resulta en la obtención de dichos cristales, y sus aplicaciones dependerán de la estructura cristalina de los mismos. Este trabajo se encuentra centrado en el estudio de la levadura Candida krusei al ser expuesta de manera simultánea a iones de los metales pesados Pb 2+ y Cd 2+ en diferentes relaciones. La susceptibilidad de la cepa a dichas mezclas de metales tóxicos fue evaluada y se inició la caracterización y purificación de los cristales obtenidos.

Photo-conducting CdS films were coated on glass at 450 °C using cadmium chloride and thiourea as Cd and S sources, respectively, with different concentrations. The sprayed CdS films are crystallized in the hexagonal structure and orienting along (0 0 2) plane with good adherence. All the films have high optical absorption in the visible region showing optical bandgap values in the range of 2.39-2.43 eV. The variation of precursor alters the surface morphology of the films. The formed grains are uniformly spread over the substrate and highly agglomerated at 0.15 M concentration. Band to band emission and defect-related emission are reported using photoluminescence (PL) measurements. The CdS device shows relatively high photosensitivity of 0.4 A/W, detectivity of 8.46 × 10 10 Jones, external quantum efficiency (EQE of 140%) with a rise time about 0.2 s and decay time about 0.3 s. These results propose that the CdS thin films are potential candidates for the visible photo-detector applications prepared using an easy and low-cost fabrication method.

Anisotropic optical constants of N-layer ReS2 are determined by angle resolved reflection measurements. Optimum parameters for a metal nanoparticle array leading to maximum light-matter interaction are determined using numerical simulations. Plasmonic enhancement in absorptance of the ReS2 layer and photocurrent are demonstrated experimentally.

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry. * Corresponding authors: matt_beard@nrel.gov, anozik@nrel.gov. † These authors contributed equally to this work.

A sharp response of infrared light detecting capability is observed in reduced graphene oxide based photodetector. The photoresponse increased with increasing solution concentration and annealing duration, at a low percolation. The photocurrent increases considerably before saturation and persists for a maximum time of 26 min. This Letter also reports on superiority of device performance with continuous annealing over annealing with an interval. A maximum photoresponsivity of 0.55 A/W and external quantum efficiency of 57% was achieved at low power light intensity (0.8 mW/cm 2 ). This Letter presents an efficient graphene oxide thin film infrared photodetector processed via a modified synthesis protocol.

We have studied the optical absorptance of lead sulfide (PbS) quantum dot (QD) coated thin crystalline layered material (TCLM) experimentally and numerically. Starting with the synthesis, fabrication and characterizations, a sample of PbS QDs deposited on trilayer molybdenum disulphide (MoS 2) thin film has been probed locally using a reflection spectrometer setup. Since transmittance is needed to calculate the absorbtance of the QD film/TCLM sample, we run a set of simulations using a 3D finite-difference time-domain full-wave electromagnetic solver. Based on the agreement between experimental and numerical results for the reflectance spectra, which verifies the accuracy of our QD and TCLM modelling, we have calculated the absorptance. Unlike metal nanoparticle decorated TCLMs in which metal nanoparticles act like induced dipoles and enhance the absorptance, here we have not observed the similar effect; rather we have found that the absorptance of QD film/TCLM sample is almost equal to the summation of QDs " and TCLM " s individual absorptance in the wavelength range of 450-800 nm.

In this work we explore the preparation of complex-shaped semiconductor nanostructures composed of different materials via a cationic exchange process in which the cations of the original semiconductor nanostructure are replaced by cations of different metals with preservation of the shape and the anionic framework of the nanocrystals. Utilizing this cation exchange method, we synthesized two new tetrapods for the first time: Cu 2−x Se/Cu 2−x S and PbSe/PbS, both prepared from CdSe/CdS tetrapods as 'templates'. We also fabricated near-infrared (NIR) photodetectors with a very simple architecture comprising a PbSe/PbS tetrapod layer between two Au electrodes on a glass substrate. When illuminated by a NIR laser, these devices are capable of achieving a responsivity of 11.9 A W −1 without the use of ligand-exchange processes, thermal annealing or hybrid device architecture. Transient absorption spectroscopy was carried out on these PbSe/PbS tetrapods, the results of which suggest that the branched morphology contributes in part to device performance. Investigation of the charge dynamics of the PbSe/PbS tetra-pods revealed an extremely long-lived exciton recombination lifetime of ∼17 ms, which can result in enhanced photoconductive gain. Overall, these heterostructured tetrapods showcase simultaneously the importance of nanoparticle shape, band structure, and surface chemistry in the attainment of NIR

TiO 2 nanoparticles (TiO 2 NPs) were prepared by pulsed ablation in liquid for ablation of a titanium target immersed in deionised water, while porous silicon (PS) is synthesised by photoelectrochemical etching (PECE) of n-type silicon. TiO 2 NPs colloidal solution is deposited on PS by drop casting technique. The X-ray diffraction techniques and scanning electron microscopy confirm the formation of anatase TiO 2 NPs with 36–124 nm size. Atomic force microscopy shows that PS has sponge-like structure with 14.18 nm average pore diameter. Capacitance–voltage (C–V) measurement of Al/TiO 2 NPs/PS/n-Si/Al detector revealed an abrupt junction. Al/TiO 2 NPs/PS/n-Si/Al detector has showed low reflectivity with good responsivity of 0.053 A/W and 2.08 × 10 12 W −1 cm Hz 1/2 specific detectivity.

We numerically study the possibility of using atomically thin transition metal dichalcogenides (TMDs) for applications requiring broadband absorption in the visible range of the electromagnetic spectrum. We demonstrate that when monolayer TMDs are positioned into a finite-period of multilayer Bragg stack geometry, they make broadband, wide-angle, almost polarization-independent absorbers. In our study, we consider molybdenum disulfide (MoS 2) and silicon dioxide (SiO 2) as semiconducting and dielectric thin film of alternate high-and low-index films, respectively. By optimizing the thickness of the SiO 2 film, we find that monolayer MoS 2 based Bragg stacks can absorb 94.7% of the incident energy in the visible (350–700 nm). Similar structures can be engineered to make perfect reflectors for saturable absorption applications. We also demonstrate that bandwidth of metamaterial absorbers can be expanded using monolayer TMDs.

Unabated use of fossil fuels continues to drive our atmospheric concentration of CO 2 toward unacceptable levels. Development of tremendous quantities of carbon-free energy would circumvent the potentially disastrous problems associated with this human-generated climate change. Sunlight represents a carbonfree source of renewable energy capable of far exceeding global power demands. The sun's radiative power at Earth's surface provides almost as much energy in one hour as the total energy consumed on Earth in one year. 1 However, to generate economically competitive solar-based electricity and/or fuels, photovoltaic (PV) cells need to be both much less expensive than they are now and significantly more efficient than the ∼11-14% sunlight-to-electricity conversion efficiency achieved by conventional silicon-based cells.

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 ( 0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 ( 5% and 80 ( 7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 ( 0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 ( 5% and 80 ( 7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.

Unabated use of fossil fuels continues to drive our atmospheric concentration of CO 2 toward unacceptable levels. Development of tremendous quantities of carbon-free energy would circumvent the potentially disastrous problems associated with this human-generated climate change. Sunlight represents a carbonfree source of renewable energy capable of far exceeding global power demands. The sun's radiative power at Earth's surface provides almost as much energy in one hour as the total energy consumed on Earth in one year. 1 However, to generate economically competitive solar-based electricity and/or fuels, photovoltaic (PV) cells need to be both much less expensive than they are now and significantly more efficient than the ∼11-14% sunlight-to-electricity conversion efficiency achieved by conventional silicon-based cells.

Spin-cast all-inorganic nanoparticle solutions have been used to make a CdTe/CdSe solar cell with an efficiency of up to 2.6% without alumina or calcium buffer layers. The type of junction as well as the non-selective nature of the electrodes of these devices are explored.

A b s t r a c t We h a v e s t u d i e d c h a r a c t e r i z a t i o n o f micromechanical cleavage monolayer molybdenum disulfide (MoS 2 ) deposited on SiO 2 /Si and glass substrates. Spherical gold (Au) nanoparticles were anchored on top of monolayer MoS 2 film. Direct reflection spectra were measured, which infers significant improvement of total average absorption of monolayer MoS 2 in Vis-NIR range of 400-800 nm by the plasmonic absorption of Au nanoparticles. Nearly 8 % average absorption was obtained in monolayer MoS 2 sample, which was increased up to 41 % by plasmonic Au nanoparticles deposited on monolayer MoS 2 film. Numerical results are also provided to describe the phenomena. This study will further add the possibilities of using layered materials in the area of solar absorptionbased nano-optoelectronic devices.

Nanoparticles of PbS are grown by simple chemical reduction route using THF as capping agent. The Brovine Serum Albumin is dissolved in water. PbS nanoparticles are also dissolved in water. The dissolved protein is mixed with dispersed nanoparticles. The shifting of optically absorbed protein is observed.