Exciton Peak Research Articles

Layer Rotation-Angle-Dependent Excitonic Absorption in van der Waals Heterostructures Revealed by Electron Energy Loss Spectroscopy.

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. The optoelectronic functionality of 2D heterostructure devices is critically dependent on the relative rotation angle of the layers. However, the role of the relative rotation angle of the constituent layers on intralayer absorption is not clear yet. Here, we investigate MoS2/WSe2 vdW heterostructures using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy and report that momentum conservation is a critical factor in the intralayer absorption of TMDC vdW heterostructures. The evolution of the intralayer excitonic low-loss EEL spectroscopy peak broadenings as a function of the rotation angle reveals that the interlayer charge transfer rate can be about an order of magnitude faster in the aligned (or anti-aligned) case than in the misaligned cases. These results provide a deeper insight into the role of momentum conservation, one of the fundamental principles governing charge transfer dynamics in 2D vdW heterostructures.

Structural, optical and photocatalytic properties of ZnS/zeolite Y nanoparticles synthesized by γ-ray irradiation

The present study deals with würtzite ZnS nanoparticles embedded in zeolite Y, synthesized by γ-radiolysis of exchanged zeolite ZnY in the presence of 2-mercaptoethanol (RSH). The nanocomposites were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), UV–Visible absorption, and photoluminescence (PL). Compared to the bulk state, ZnS nanocrystals have blue – shifted UV visible absorption with an excitonic peak situated at around 320 nm. They display a wide photoluminescence band with seven peaks in the range of 320–550 nm. The increase of the irradiation dose and RSH concentration results in a growth the ZnS nanocrystals and improves their optical performances. The optical as well as the structural characterizations confirmed the nanometric size of ZnS particles (Φ = 5–18 nm). When tested in photodegradation of Congo red under sun light, ZnS-zeolite Y catalyst exhibit high efficiency. For the experimental conditions: [ZnS-Y] = 0.5 g L−1; [CR] = 10 mgl−1+; pH = 6.6, the pseudo-first order constant is 0.076 min−1.

Fluorescence Invigoration in Carbon-Incorporated Zinc Oxide Nanowires from Passage of Field Emission Electrons

We demonstrate that carbon incorporated Zinc Oxide (C-ZnO) nanowires (NWs) exhibit remarkable improvement in the extent and quality of fluorescence emission after they are utilized as an electron source in a field emission experiment. After the passage of field emission electrons, the intensity of the fluorescence emitted from these NWs in the visible light range exhibits a 2.5 to 8 fold enhancement. The intrinsic exciton peak of the ZnO also becomes heightened, along with the crystalline quality of the NWs showing marked improvement. This invigoration of fluorescence across the entire fluorescence spectrum is attributed to concurrent removal of oxygen and carbon atoms in C-ZnO NWs due to electro-migration of atoms and joule heating during the field emission process. Applications based on ZnO NWs emission from excitonic emissions or visible wavelength emissions or both can benefit from this straightforward method of defect engineering.

Electronic and optical properties of TiO2 by first-principle calculation (DFT-GW and BSE)

In this, we present improved measures for the characterization of TiO2. The electronic and optical properties of TiO2 are calculated employing the density functional theory (DFT) within local density approximation (LDA) and also many-body perturbation theory (MBPT) based on quasiparticle GW approximation. The bandgap is significantly improved from the conventional use of DFT. The optical spectra are calculated by solving the Bethe-Salpeter equation (BSE) with regard to the exciton correlation effect, which shows strong excitonic peaks. Our BSE results show a greater agreement between theory and experimental data than previous methods.

Сolloidal synthesis and characterization of hydrophilic CdTe quantum dots for medical diagnostics

The present article deals with colloidal quantum dots (QDs) synthesized in an aqueous medium using thioglycolic acid (TGA), L-cysteine (L-cys) and mercaptoethylamine (MEA) as stabilizers. In contrast to high-temperature synthesis in an organic medium, this method of synthesis enables us to skip an additional time-consuming stage of hydrophilization of QDs. The resulting CdTe QDs were studied by spectroscopic methods of analysis. In the absorption spectra of the QDs there is an exciton peak which corresponds to the minimum energy required to form excitons in the QDs. The average size of the QDs, which is about 3-4 nm, was calculated based on the position of the exciton peak. The zeta potential of the QDs was measured and the results were compared with literature data.

Electronic and optical excitations of two-dimensional ZrS2 and HfS2 and their heterostructure

In a first-principles study based on density-functional theory and many-body perturbation theory we investigate the electronic properties and the optical excitations of ZrS$_2$ and HfS$_2$ monolayers and their van der Waals heterostructure. Both materials have an indirect quasi-particle band gap, which amounts to about 2.8 eV in ZrS$_2$ and to 2.6 eV in HfS$_2$. In both systems the valence-band maximum is at $\Gamma$ and the conduction-band minimum at M. Spin-orbit coupling induces a splitting of about 100 meV at the $\Gamma$ point in the valence band, while it does not affect the conduction band. The optical absorption spectra are dominated by excitonic peaks, with binding energies between 0.6 eV and 0.8 eV. The ZrS$_2$/HfS$_2$ heterobilayer exhibits a peculiar type-I level alignment with a large degree of hybridization between the two monolayers in the valence band, while the conduction bands retain either ZrS$_2$ or HfS$_2$ character, respectively. As a consequence, both the electron and the hole components of the first exciton are localized in the ZrS$_2$ monolayer with non-vanishing probability of finding the hole also in the HfS$_2$ sheet.

From Wurtzite Nanoplatelets to Zinc Blende Nanorods: Simultaneous Control of Shape and Phase in Ultrathin ZnS Nanocrystals.

Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their exciton Bohr radius receive a rapidly increasing attention due to their unique physicochemical properties. These superior properties highly depend on the shape and crystal phase of semiconductor NCs. Here, we demonstrate not only the synthesis of well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and emission, but also the shape/phase transformation between wurtzite (WZ) NPLs and zinc blende (ZB) nanorods (NRs). UV-vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp excitonic peak that is not observed in ZB-ZnS NRs. Besides, the photoluminescence characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak, while ZB-ZnS NRs exhibit a broad collective emission band consisting of four emission peaks. The appearance of excitonic features in the absorption spectra of ZnS NPLs is explained by interband electronic transitions, which is simulated in the framework of atomic effective pseudopotentials (AEP).

Photoluminescence of CuInSe2/GaN and CuInSe2/InN

The power and temperature dependent photoluminescence (PL) of epitaxially grown In-rich CuInSe2 (CIS) and Cu-rich CIS deposited on N-polar GaN and InN were investigated in this paper. The In-rich CIS/GaN has two PL emissions characterized by a donor-acceptor pair (DAP) peak at 0.92 eV and an excitonic peak at 1.08/1.1 eV. On the other hand, the Cu-rich CIS/GaN has four PL emissions characterized by two DAP peaks at 0.86 eV and 0.94 eV, free-to-bound recombination peak at 0.97 eV and an excitonic peak at 1.03 eV. Identification of these defects in the CIS absorber layer is crucial for the improvement of the device efficiency. The PL emission of the CIS/InN closely resembles that of the Cu-rich CIS/GaN. For all the samples, the PL intensity increased with excitation power while the PL intensity decreased with temperature. The obtained power coefficients and activation energies support the proposed mechanism causing the luminescence. The defects present in CIS/GaN and CIS/InN were also identified. The results from this study are consistent with those of CIS single crystals and show that the CIS absorber layer can be incorporated with III-nitride materials, and its absorption can be extended beyond the spectrum covered by plain CIS.

Probing the Optical Properties of MoS2 on SiO2/Si and Sapphire Substrates.

As an important supplementary material to graphene in the optoelectronics field, molybdenum disulfide (MoS2) has attracted attention from researchers due to its good light absorption capacity and adjustable bandgap. In this paper, MoS2 layers are respectively grown on SiO2/Si and sapphire substrates by atmospheric pressure chemical vapor deposition (APCVD). Atomic force microscopy, optical microscopy, and Raman and photoluminescence spectroscopy are used to probe the optical properties of MoS2 on SiO2/Si and sapphire substrates systematically. The peak shift between the characteristic A1g and E12g peaks increases, and the I peak of the PL spectrum on the SiO2/Si substrate redshifts slightly when the layer numbers were increased, which can help in obtaining the layer number and peak position of MoS2. Moreover, the difference from monolayer MoS2 on the SiO2/Si substrate is that the B peak of the PL spectrum has a blueshift of 56 meV and the characteristic E12g peak of the Raman spectrum has no blueshift. The 1- and 2-layer MoS2 on a sapphire substrate had a higher PL peak intensity than that of the SiO2/Si substrate. When the laser wavelength is transformed from 532 to 633 nm, the position of I exciton peak has a blueshift of 16 meV, and the PL intensity of monolayer MoS2 on the SiO2/Si substrate increases. The optical properties of MoS2 can be obtained, which is helpful for the fabrication of optoelectronic devices.

Inhomogeneous Broadening of the Exciton Band in Optical Absorption Spectra of InP/ZnS Nanocrystals.

In this work, we have simulated the processes of broadening the first exciton band in optical absorption spectra (OA) for InP/ZnS ensembles of colloidal quantum dots (QDs). A phenomenological model has been proposed that takes into account the effects of the exciton–phonon interaction, and allows one to analyze the influence of the static and dynamic types of atomic disorder on the temperature changes in the spectral characteristics in question. To vary the degree of static disorder in the model system, we have used a parameter δ, which characterizes the QD dispersion in size over the ensemble. We have also calculated the temperature shifts of the maxima and changes in the half-width for the exciton peaks in single nanocrystals (δ = 0), as well as for the integrated OA bands in the QD ensembles with different values of δ = 0.6–17%. The simulation results and the OA spectra data measured for InP/ZnS nanocrystals of 2.1 nm (δ = 11.1%) and 2.3 nm (δ = 17.3%), are in good mutual agreement in the temperature range of 6.5 K–RT. It has been shown that the contribution of static disorder to the observed inhomogeneous broadening of the OA bands for the QDs at room temperature exceeds 90%. The computational experiments performed indicate that the temperature shift of the maximum for the integrated OA band coincides with that for the exciton peak in a single nanocrystal. In this case, a reliable estimate of the parameters of the fundamental exciton–phonon interaction can be made. Simultaneously, the values of the specified parameters, calculated from the temperature broadening of the OA spectra, can be significantly different from the true ones due to the effects of static atomic disorder in real QD ensembles.

Physical investigations and DFT model calculation on Zn2SnO4-ZnO (ZTO-ZO) alloy thin films for wettability and photocatalysis purposes

This work covers some physical investigations on zinc-tin-oxide alloy thin films deposited on glass substrates at 450 °C by the spray pyrolysis technique. The effect of annealing in air on the physical properties of such films has been carried out. First, XRD analysis revealed good crystallinity of Zn-Sn-O as deposited film with presence of both Zn2SnO4 and ZnO phases. This was further supported by room temperature photoluminescence measurements showing the excitonic peaks for both phases, in addition to other peaks that are mainly due to both oxygen and zinc related defects. Second, the geometry optimization and the electronic properties of Zn2SnO4 were carried out using density functional theory (DFT) with corrected exchange correlation functional (PBE0) as implemented in the CRYSTAL14 code. The density of states indicates that the top of valence band is mainly built upon O-2p states whereas the bottom of the conduction band principally originates from Zn and Sn-5p states. Ab initio optimized structural parameters and electronic band structure conducted in this work shows a good agreement with our experimental data. Moreover, the hydropholic investigations have been carried out in terms of water contact angle (WCA) and reveal that these films have a hydropholic nature. Finally, the photocatalytic performances of both as deposited and annealed Zn-Sn-O thin films ones were investigated by degradation of aqueous methylene blue dye solutions under sunlight illumination for five hours. The results may be of interest for future sensitivity appkications like gas and bio-sensors.

(Invited) A Review of Defects in 2D Metal Dichalcogenides: Doping, Alloys, Interfaces, Vacancies and Their Effects in Catalysis & Optical Emission

Two-dimensional transition metal dichalcogenides (TMDs) such as MoS2 and WS2 hold great promise for many novel applications. Recent years have therefore witnessed tremendous efforts on large scale manufacturing of these 2D crystals. A long-standing puzzle in the field is the effect of different types of defects in their electronic, magnetic, catalytic and optical properties. In this presentation an overview of different defects in transmission metal di-chalcogenides (TMDs) will be presented [1,2]. We will first focus on: 1) defining the dimensionalities and atomic structures of defects; 2) pathways to generating structural defects during and after synthesis and, 3) the effects of having defects on the physico-chemical properties and applications. We will also emphasize doping and allowing monolayers of MoS2 and WS2, and their implications in electronic and thermal transport. We will also describe the catalytic effects of edges, vacancies and local strain observed in MoxW(1-x)S2 monolayers by correlating the hydrogen evolution reaction (HER) with aberration corrected scanning transmission electron microscopy (AC-HRSTEM) [3]. Our findings demonstrates that it is now possible to use chalcogenide layers for the fabrication of more effective catalytic substrates, however, defect control is required to tailor their performance. By studying photoluminescence spectra, atomic structure imaging, and band structure calculations, we also demonstrate that the most dominating synthetic defect—sulfur monovacancies in TMDs, is responsible for a new low temperature excitonic transition peak in photoluminescence 300 meV away from the neutral exciton emission [4]. We further show that these neutral excitons bind to sulfur mono-vacancies at low temperature, and the recombination of bound excitons provides a unique spectroscopic signature of sulfur mono-vacancies [4]. However, at room temperature, this unique spectroscopic signature completely disappears due to thermal dissociation of bound excitons [4]. Finally, hetero-interfaces in TMDs, will be studied and discussed by AC-HRSTEM and optical emission.

Narrow Band-Edge Photoluminescence of Ga3+-Doped AgInS2 Quantum Dots

Quantum dots (QDs) composed of multinary I-III-VI semiconductors and related materials exhibit unique composition- and size-dependent physicochemical properties and have attracted much attention for the application to luminescent devices, solar cells, and photocatalyts because of their strong absorption coefficient and low toxicity. It has been reported that the QDs composed of CuInS2, AgInS2 and their solid solution with ZnS exhibited strong photoluminescence (PL) in the visible light wavelength region, PL peak wavelength being tunable by controlling the particle size and composition. Most of these multinary QDs exhibited broad defect-site PL peaks with a large Stokes shift, which may limit large-scale commercial applications. In contrast, recently, we have reported that rod-shaped (AgIn)xZn2(1-x)Te2 QDs with almost stoichiometric composition exhibited a clear exciton peak in the absorption spectra and a narrow band-edge emission peak in the near-IR wavelength region, the peak wavelengths of which were blue-shifted with a decrease in the x value.(1) In this study, we synthesize non-stoichiometric Ag-In-Ga-S (AIGS) QDs and then investigate their PL properties in relation to their chemical compositions.(2,3) AIGS QDs were synthesized in a mixture solvent of oleylamine and dodecanethiol by the thermal decomposition of the precursor mixture of elemental sulfur, Ag(OAc), Ga(acac)3, and In(acac)3 at 300 oC for 10 min with vigorous stirring under an N2 atmosphere, in which the ratios of Ag/(Ag+In+Ga) and In/(In+Ga) were varied. By adding methanol to the resulting solution, the target AIGS nanoparticles were isolated as wet precipitates. GaSx shell coating was carried out by the heat treatment of AIGS QDs in oleylamine in the presence of Ga(acac)3 and thiourea. Stoichiometric AIGS QDs, which had Ag/(Ag+In+Ga)= 0.5, exhibited a broad defect-site PL peak with a small narrow band-edge PL peak. With a decrease in the Ag/(Ag+In+Ga) ratio, the intensity of band-edge peak was enlarged, being optimal at non-stoichiometric composition of Ag/(Ag+In+Ga)= ca. 0.4. The wavelength of band-edge PL peak was blue-shifted from 610 to 500 nm by decreasing the content of In in AIGS QDs from In/(In+Ga)= 1.0 to 0.2. The GaSx coating of AIGS QDs produced amorphous shell layer of ca. 1 nm thickness on AIGS core surface. Thus-obtained AIGS@GaSx core-shell particles predominantly exhibited a band-edge PL peak of 41 nm in FWHM, and then the intensity of broad defect-site peak was significantly reduced to less than 15 %. The optimal PL quantum yield of AIGS@GaSx core-shell QDs was 28% with green band-edge emission at 530 nm. The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic element-free I-III-VI-based QDs in a wide area of applications. Reference (1) T. Kameyama et al., J. Mater. Chem. C 2018, 6, 2034. (2) T. Uematsu et al., NPG Asia Materials 2018, 10, 713 (3) T. Kameyama et al., ACS Appl. Mater. Interfaces 2018 , 10, 42844.

Modulation of New Excitons in Transition Metal Dichalcogenide-Perovskite Oxide System.

The exciton, a quasi‐particle that creates a bound state of an electron and a hole, is typically found in semiconductors. It has attracted major attention in the context of both fundamental science and practical applications. Transition metal dichalcogenides (TMDs) are a new class of 2D materials that include direct band‐gap semiconductors with strong spin–orbit coupling and many‐body interactions. Manipulating new excitons in semiconducting TMDs could generate a novel means of application in nanodevices. Here, the observation of high‐energy excitonic peaks in the monolayer‐MoS2 on a SrTiO3 heterointerface generated by a new complex mechanism is reported, based on a comprehensive study that comprises temperature‐dependent optical spectroscopies and first‐principles calculations. The appearance of these excitons is attributed to the change in many‐body interactions that occurs alongside the interfacial orbital hybridization and spin–orbit coupling brought about by the excitonic effect propagated from the substrate. This has further led to the formation of a Fermi‐surface feature at the interface. The results provide an atomic‐scale understanding of the heterointerface between monolayer‐TMDs and perovskite oxide and highlight the importance of spin–orbit–charge–lattice coupling on the intrinsic properties of atomic‐layer heterostructures, which open up a way to manipulate the excitonic effects in monolayer TMDs via an interfacial system.

Photoinduced Atomic Force Spectroscopy and Imaging of Two-Dimensional Materials

Correlating surface features and the resultant evolution of optical properties is important in the study and application of two-dimensional (2D) materials like transition-metal dichalcogenides. The authors measure photoinduced forces in atomically thin layers of MoS${}_{2}$ and WS${}_{2}$ using a modified atomic force microscope, enabling their visualization with high contrast. A spectral-force retrieval technique enables evaluation of the distinct excitonic peaks of the two structures, which resemble the excitonic features of far-field absorption. Spectral signatures of defects and edges are also observed, opening the way toward nondestructive, high-resolution defect imaging of 2D materials.

Exciton absorption spectra in narrow armchair graphene nanoribbons in an electric field

We present an analytical investigation of the exciton optical absorption in a narrow armchair graphene nanoribbon (AGNR) in the presence of a longitudinal external electric field directed parallel to the ribbon axis. The two-body 2D Dirac equation for the massless electron and hole subject to the ribbon confinement, Coulomb interaction and electric field is employed. The ribbon confinement is assumed to be much stronger than the internal exciton electric field which in turn considerably exceeds the external electric field. This justifies an adiabatic approximation implying a slow longitudinal and fast transverse electron-hole relative motion governed by the exciton attraction and accompanied by the external electric field and ribbon confinement effects, respectively. In the single subband approximation of the isolated size-quantized subbands induced by the ribbon confinement the exciton electroabsorption coefficient is determined in an explicit form. The pronounced dependencies of the exciton peak positions, widths and intensities on the ribbon width and electric field strength are traced. The electron-hole exciton attraction modifies remarkably the Franz-Keldysh electroabsorption in the frequency region both below and above the edges determined by the size-quantized energy levels. In the double-subband approximation of the interacting ground and first excited subbands the combined effect of the exciton electro- and autoionization caused by the electric field and intersubband coupling, respectively, are explored. Estimates of the expected experimental values for the typically employed AGNR show that for a weak electric field the exciton quasi-discrete states remain sufficiently stable to be observed in optical experiments, while relatively strong fields free the captured carriers to further restore their contribution to the transport.

Giant exciton-phonon coupling and zero-point renormalization in hexagonal monolayer boron nitride

We report here a giant zero-point energy renormalization of 273 meV in the direct band gap at $\mathbf{K}$ in the Brillouin zone and a 571 meV of blueshifting in the position of the doubly-degenerate brightest excitonic peak in monolayer hexagonal boron nitride. The nonradiative exciton linewidth is found to be 97 meV at 0 K with a large coupling strength of 1.1 eV. This linewidth is found to be mainly dominated by the scattering from the longitudinal optical phonons near the degenerate LO-TO mode, with negligible contributions from other lower branches. Additionally, the band gap has a temperature dependent slope of $\ensuremath{-}0.53$ meV ${\mathrm{K}}^{\ensuremath{-}1}$, which we found to be in excellent agreement with the reported experimental data on large diameter boron nitride nanotubes. We obtained our results by solving a coupled electron-hole Bethe-Salpeter equation which includes the lattice vibrational dynamics, purely using an ab initio approach.

Influence of Nd3+ doping in ZnO nanoparticles to enhance the optical and photocatalytic activity

Zn1−xNdxO (x = 0.0, 0.03, 0.06, 0.09, 0.12, 0.15) nanoparticles have been synthesized using simple chemical precipitation techniques using hydroxyoxalate type precursors. X-Ray Diffractogram (XRD) of the compounds reveals hexagonal wurtzite structure in all the compositions with crystallite size around 50 nm. Grain size and growth direction of the specimen have been identified by using High Resolution Transmission Electron Microscopy (HRTEM). Energy Dispersive x-ray (EDX) spectroscopy confirms the doping of Nd in predictable concentration. UV–vis absorption spectra indicate an increase in the optical band gap with the increase in Nd concentration in ZnO. A strong excitonic peak at 367 nm corresponding to near band edge emission is substantiated through Fluorescence (FL) spectra for Nd3+ doped ZnO nanoparticles. Enhanced photocatalytic activity has been observed with the increase in concentration of Nd in the doped ZnO nanoparticles.

Possible evidence of delocalized excitons in Cr-doped PrFeO3: An experimental and theoretical realization

Here we introduce a way to understand the phenomena of localization/delocalization of excitons experimentally as well as by first-principle study. Using in-situ temperature-dependent optical absorption spectroscopy (TD OAS), variation of delocalization parameter (ξ) and electronic disorder in terms of Urbach energy as a function of temperature was estimated. For this purpose, polycrystalline samples of PrFe1−xCrxO3 (x = 0 to 0.25) were synthesized via wet chemical method. The ξ increased with inclusion of Cr doping, which could be understood in terms of an increase in delocalized nature of excitons. Further, the theoretically simulated optical absorption spectrum for undoped and Cr-doped PrFeO3 clearly indicated the change in excitonic peak from almost Gaussian to Lorentzian nature and was explained in terms of an increase in delocalized nature of excitons with inclusion of Cr doping. Thus, using the combination of TD OAS and first-principle calculations, we demonstrated a way to acquire precise information about exciton dynamics, which could be very helpful in understanding many interesting phenomena such as electron–phonon coupling, electronic disorder near band edge and transport properties.

A Facet-Specific Quantum Dot Passivation Strategy for Colloid Management and Efficient Infrared Photovoltaics.

Colloidal nanocrystals combine size- and facet-dependent properties with solution processing. They offer thus a compelling suite of materials for technological applications. Their size- and facet-tunable features are studied in synthesis; however, to exploit their features in optoelectronic devices, it will be essential to translate control over size and facets from the colloid all the way to the film. Larger-diameter colloidal quantum dots (CQDs) offer the attractive possibility of harvesting infrared (IR) solar energy beyond absorption of silicon photovoltaics. These CQDs exhibit facets (nonpolar (100)) undisplayed in small-diameter CQDs; and the materials chemistry of smaller nanocrystals fails consequently to translate to materials for the short-wavelength IR regime. A new colloidal management strategy targeting the passivation of both (100) and (111) facets is demonstrated using distinct choices of cations and anions. The approach leads to narrow-bandgap CQDs with impressive colloidal stability and photoluminescence quantum yield. Photophysical studies confirm a reduction both in Stokes shift (≈47 meV) and Urbach tail (≈29 meV). This approach provides a ≈50% increase in the power conversion efficiency of IR photovoltaics compared to controls, and a ≈70% external quantum efficiency at their excitonic peak.