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Transition metal quantum dots composites for an electrocatalytic hydrogen evolution reaction.

An intense increase in non-biodegradable plastics and waste metals is an immediate threat to the world and needs to be addressed urgently.

Over the last few years, photocatalysis using solar radiation has been explored extensively to investigate the possibilities of producing fuels. The production and systematic usage of solar fuels can reduce the use of fossil-based fuels, which are currently the primary source for the energy. It is time for us to exploit renewable sources for our energy needs to progress towards a low-carbon society. This can be achieved by utilizing green hydrogen as the future energy source. Solar light-assisted hydrogen evolution through photocatalytic water splitting is one of the most advanced approaches, but it is a non-spontaneous chemical process and restricted by a kinetically demanding oxidation evolution reaction. Sunlight is one of the essential sources for the photoreforming (PR) of biomass waste into solar fuels, or/and lucrative fine chemicals. Hydrogen production through photoreforming of biomass can be considered energy neutral as it requires only low energy to overcome the activation barrier and an alternate method for the water splitting reaction. Towards the perspective of sustainability and zero emission norms, hydrogen production from biomass-derived feedstocks is an affordable and efficient process. Widely used photocatalyst materials, such as metal oxides, sulphides and polymeric semiconductors, still possess challenges in terms of their performance and stability. Recently, a new class of materials has emerged as organic-inorganic hybrid (OIH) photocatalysts, which have the benefits of both components, with peculiar properties and outstanding energy conversion capability. This work examines the most recent progress in the photoreforming of biomass and its derivatives using OIHs as excellent catalysts for hydrogen evolution. The fundamental aspects of the PR mechanism and different methods of hydrogen production from biomass are discussed. Additionally, an interaction between both composite materials at the atomic level has been discussed in detail in the recent literature. Finally, the opportunities and future perspective for the synthesis and development of OIH catalysts are discussed briefly with regards to biomass photo-reforming.

Photo-current characteristics in close space sublimation processed multi-facetted Cadmium Telluride (CdTe) micro-granular films were studied under AM1.5, blue and green laser illuminations to assess the energy dependent charge transport characteristics. Photo-current under AM1.5 illumination (100 mW/cm 2 ) resulted in three times more than dark current while it was observed 2.4 and 2.5 times more in case of blue and green laser illuminations, respectively. While all the three illuminations (AM1.5, blue and green lasers) were capable to generate photo-electrons in CdTe, the variation in magnitude was articulated to the effect of grain boundaries present across the charge transport pathways as evidenced from the surface morphology examined by scanning electron microscopic studies which asserted that CdTe films were micro-granular in nature separated by grain boundaries. It is elucidated in the measurements that charge carriers under the illumination of AM1.5 light, acquired energy to overcome the grain boundaries easily compared to the charge carriers produced by blue and green light sources as evidenced from the photo-current measurements.

A dominant interfacial recombination pathway in the dye sensitized solar cell (DSSC) was suppressed by coating graphene oxide (GO) on the titanium dioxide (TiO2) nanoparticle layer via the electrophoretic deposition (EPD) method. DSSC utilizing 5 min coating of GO by EPD on TiO2 nanoparticle layer showed 5% enhancement in the photo-conversion efficiency (from 5.7% to 6.0%), and 5% enhancement in the short circuit current density (from 11.4 mA/cm2 to 12.0 mA/cm2) in comparison with reference DSSCs which did not use GO on TiO2. GO coating on TiO2 is attributed to the efficient suppression of the photo-generated electron–hole recombination at the TiO2/dye/electrolyte interfaces. The further increase in the thickness of GO (10- and 20-min EPD coating) on the TiO2 nanoparticle layer impeded the charge transport as the performance of the respective DSSCs was significantly affected. It suggested that the probability of photo-generated electron tunneling from dye to TiO2 was suppressed by increasing the thickness of the GO layer. Presented results assure that GO can be considered as a competitive surface passivation candidate for nanostructured excitonic solar cells.

Photo-generated charge carrier recombination in dye-sensitized solar cells (DSSCs) is observed to be suppressed significantly at the interface between transparent fluorine-doped tin oxide (FTO) and titanium dioxide (TiO2) by coating nickel oxide (NiO) thin film by RF sputtering. UV-Visible optical absorption spectroscopic measurements performed in the wavelength window of 300–800 nm showed ~ 60% average transmittance for NiO thin films coated for 10 min. The calculated optical bandgap value for NiO was 3.4 eV. The RF-sputtered NiO films were thoroughly characterized by X-ray photo-electron spectroscopy to examine Ni 2p3/2 and Ni 2p1/2 along with O 1s. The present study assessed the effect of 5, 10, and 15 min RF-sputtered NiO thin films at the interface between FTO and mesoporous TiO2. Results showed that charge transport in DSSCs is highly sensitive to NiO thickness at the interface between FTO and TiO2. It was specifically noticed that 10 min coating of NiO on FTO yielded DSSCs with photo-conversion efficiency (η) of ~ 6.8% while DSSCs with no NiO on FTO showed only 4.9%. Further increase in NiO thickness affected the performance of DSSCs due to the significant reduction in tunneling probability from TiO2 to FTO.

Nanoflakes of two-dimensional (2D)-layered molybdenum diselenide (MoSe2) were used in zinc oxide (ZnO)-based dye-sensitized solar cells (DSSCs) to improve the charge transport by minimizing the recombination loss. The present study demonstrates that photovoltaic performance of the DSSCs can be improved up to ~ 87%. Introducing MoSe2 into ZnO in DSSCs integrates benefits in terms of both electrical and optical as it improved the absorption and charge transport. MoSe2 nanoflakes into nanostructured ZnO did not change anything significantly in terms of surface morphology as they are atomically thin. Specifically, incorporation of 0.65 weight % of MoSe2 into ZnO increased short-circuit current density of the DSSC by ~ 70% (from 4.7 to 8 mA.cm−2) due to an effective optical absorption in the spectral range of 350–800 nm with two characteristic peaks at 550 nm and 670 nm. Further, it was also attributed to the presence of MoSe2 flakes in ZnO, which constituted efficient charge transport pathways, in terms of dangling-bond free surface, through which electron transport occurred with minimum possible loss. It was also noticed that variation in MoSe2 quantity in ZnO plays a critical role in DSSC performance.

High-temperature annealing in tungsten disulfide resulted in heterogeneous WS2–WO3 in which intra- (within WS2 and WO3) and inter- (between WS2 and WO3) grain boundaries were observed, which were highly critical for charge transport and recombination. The heterogeneous WS2–WO3 phase was evidenced by observing the coexistence of d-spacing values of 0.26 nm (WS2) and 0.37 nm (WO3) in transmission electron microscopic (TEM) studies. Further systematic high-resolution TEM studies elucidated that intra-grain boundaries separated crystallites within WS2 and WO3, while inter-grain boundaries separated WS2 from WO3. As WS2 and WO3 are both n-type, these defects are acceptor-like in the grain boundaries and they actively participate in the capture (trapping) process, which impedes charge transport characteristics in the heterogeneous WS2–WO3 films. Plasma treatment in the heterogeneous WS2–WO3 film, for 60 minutes using argon, energetically modulated the defects in the intra/inter-grain boundaries, as evidenced from detailed comparative photocurrent characteristics obtained individually in (i) pristine WS2, (ii) heterogeneous WS2–WO3 and (iii) Ar plasma-treated heterogeneous WS2–WO3 films under blue and green lasers, along with AM1.5 (1 sun) illumination. Detrimental roles (trapping/de-trapping and scattering) of grain boundary states on photoelectrons were seen to be significantly suppressed under the influence of plasma.

The present work examined dye sensitized 2D layered tungsten disulfide (WS2) as a photo-anode in solar cells with no use of nanocrystalline metal oxides as electron acceptors such as titanium dioxide. It is observed that coating WS2 directly onto a fluorine doped tin oxide (FTO) transparent conductor, annealed at 530 °C, resulted in a mixed oxy-sulfide dual phase as confirmed by transmission electron microscopy and X-ray diffraction studies. Further studies on the surface morphology of the dual phase WS2–WO3 film showed a random distribution of platelets which further shaped into precisely regulated hexagonal platelets upon plasma treatment. High resolution transmission electron microscopic studies elucidated two different phases, WS2 and WO3, with d-spacing values of 0.26 nm and 0.37 nm, respectively. A well-defined grain boundary was also observed which separated the oxy-sulfide phase in the sample. The dual WS2–WO3 phase films showed optical absorption in the wavelength range of 350 nm–800 nm with a systematic increase in plasma exposure duration. Photovoltaic devices fabricated using the WS2–WO3 mixed phase photo-anodes resulted in 0.61% efficiency (η) which further was observed to be sensitive to the plasma exposure as it was observed that the 20 minute plasma treated sample increased the η value to 0.67%. Plasma treatment on the dual-phase samples orients and modifies the shapes of the platelets with a significant change in the surface which eventually influences the charge transport in resulting photovoltaic devices and thus the variation with respect to exposure duration.

The present study examines photo-current characteristics of 2D layered molybdenum disulfide (MoS2) incorporated titanium dioxide (TiO2) nanoparticle films at different illumination conditions established using while light (AM1.5), blue (406 nm), and green (525 nm) lasers. Photo-current values, measured from a two-terminal device configuration fabricated using TiO2 nanoparticle films with different weight % of MoS2, were observed to be sensitive to the quantity of MoS2 in the bulk TiO2. It is observed that incorporation of 30 wt% of MoS2 in TiO2 enhanced photo-current up to 48.3% compared to that of pristine TiO2 nanoparticle film under AM1.5 illumination. Blue and green laser-based measurements show that the photo-current is highly sensitive to the weight fraction of MoS2 in TiO2 and incident energy used to generate excitons. The variation observed in the measured photo-current is directly attributed to the optical responsivity of MoS2 to the incoming photons.

Heterojunction solar cells are demonstrated employing spray pyrolysis-processed nano-clustered cadmium telluride (CdTe) on DC-sputtered amorphous silicon (a-Si) as p- and n-type layers, respectively. CdTe and a-Si films were subjected to X-ray photoelectron spectroscopic measurements showing Cd 3d (404.9 eV) and Te 3d (575.8 eV) peaks corresponding to CdTe while the peaks Si 2p (98.3 eV) and Si 2 s (148.3 eV) represent a-Si. Heterojunction solar cells with a stacked structure of FTO/a-Si/CdTe/Al resulted in a photo-conversion efficiency of 2.7% for an optimized thickness values of a-Si (1 µm) and CdTe (6 µm) films.

We implemented gas-phase (argon plasma) and wet-chemical (HNO3) surface treatments on close space sublimated (CSS) Cadmium Telluride (CdTe) thin films exhibiting morphologies of (i) vertically aligned walls on copper (Cu) and stainless steel (SS) substrates, (ii) valley–hills on aluminum (Al) substrates and (iii) micro-granules on nickel (Ni). As all the growth conditions (temperature, pressure, duration and source/substrate distance) were exactly the same in the CSS process to coat CdTe films, it is asserted that the various microstructures were raised only on Cu, Al, Ni and SS substrates. Plasma and HNO3 surface treatments on metal substrates did not affect the CdTe morphologies in terms of specific structures but it was observed that structural, optical and electrical charge transport characteristics were highly tunable by the two surface treatments. Thus, substrate driven morphological evaluation followed by surface treatments was enabled. The present study demonstrated various microstructures of CdTe films on metallic thin foil substrates to attempt the establishment of flexible opto-electronics based on CdTe.

Copper oxide (Cu2O) is a potential material as a catalyst for CO2 reduction. Cu2O nanostructures have many advantages, including interfacial charge separation and transportation, enhanced surface area, quantum efficiency, and feasibility of modification via composite development or integration of the favorable surface functional groups. We cover the current advancements in the synthesis of Cu2O nanomaterials in various morphological dimensions and their photochemical and electrochemical applications, which complies with the physical enrichment of their enhanced activity in every application they are employed in. The scope of fresh designs, namely composites or the hierarchy of copper oxide nanostructures, and various ways to improve CO2 reduction performance are also discussed in this review. Photochemical and electrochemical CO2 transformations have received tremendous attention in the last few years, thanks to the growing interest in renewable sources of energy and green facile chemistry. The current review provides an idea of current photochemical and electrochemical carbon dioxide fixing techniques by using Cu2O-based materials. Carboxylation and carboxylative cyclization, yield valuable chemicals such as carboxylic acids and heterocyclic compounds. Radical ions, which are induced by photo- and electrochemical reactions, as well as other high-energy organic molecules, are regarded as essential mid-products in photochemical and electrochemical reactions with CO2. It has also been claimed that CO2 can be activated to form radical anions.

The present study demonstrates the possibility for improving the performance of dye sensitized solar cell (DSSC) only by minimizing the loss in fill factor (FF) and maximum power point (PMAX) which can be achieved by passivating the nanocrystalline titanium dioxide (TiO2) using physical vapor deposited molybdenum trioxide (MoO3) thin films. The effect of MoO3 coated TiO2 on charge carrier transport was examined in resulting DSSCs and observed that ∼14% enhancement in efficiency is possible for 5 min passivation of MoO3 on TiO2. The physical vapor deposited MoO3 films were ∼75% transparent in the spectral range of 350–800 nm with an optical bandgap of ∼3.1 eV. The wide bandgap MoO3 films facilitate the incoming photons to reach the sensitizing dye to generate excitons. The 14% enhancement in the performance of DSSC by MoO3 passivation is observed through improving only the FF and PMAX while it does not contribute anything significantly to current density and open circuit voltage. Electrochemical impedance spectroscopic studies further confirmed these observations through photo-electron lifetime, which remains constant both in the bulk of pristine TiO2 and MoO3 passivated TiO2 and it further confirms the effect of MoO3 passivation on FF and PMAX in DSSCs.

Tin oxide (SnO2) nanofiber-based dye-sensitized solar cell (DSSC) exhibits an anomalous hysteresis effect on its current–voltage characteristics. While the one-dimensional nature of SnO2 nanofibers is considered facilitating effective charge transport in DSSCs, the surface states present in the SnO2 bandgap are observed to be playing an influential role in trapping and detrapping of charge carriers in the bulk as confirmed by electrochemical impedance spectroscopic studies. Forward and backward electrical bias sweeps identify the hysteresis effect in the performance of resulting DSSCs due to which 43% difference was observed in the photovoltaic performance. Charge carriers generated in the dye utilize the SnO2 nanofiber as a transport medium to reach electrode through energetically active SnO2 surface states which effectively trap and detrap the photogenerated charges during the transport resulting in the hysteresis effect. The observed hysteresis effect varies the maximum power density ( P ${}_{\rm MAX}$ ) and fill factor (FF) of the resulting DSSC by 35% and 25%, respectively, between forward and backward scans. The significant change in P ${}_{\rm MAX}$ and FF asserts that trapping and detrapping mediated charge transport process contributed to the variation in overall performance by 43%.

The present work demonstrates the possibility of facilitating electron transport in mesoscopic titanium dioxide (TiO 2 ) by incorporating nanoflakes of layered molybdenum disulfide (MoS 2 ) as an alternate electron transport bridge. Results suggest that performance of dye sensitized solar cells (DSSCs) can be increased up to ∼16% (from 7.39% to 8.55%) by incorporating 0.2 wt % of MoS 2 into the bulk of TiO 2 , due to the significant improvement in electron lifetime from 8 ms to 23 ms. The nanoflakes of MoS 2 form alternate electron transport bridges in the bulk TiO 2 nanoparticle film through which photo-injected electrons travel more efficiently to reach transparent electrode compared to DSSCs utilize only TiO 2 without MoS 2 . Presence of atomically thin layered MoS 2 nanoflakes in the bulk of TiO 2 assist the photo-electrons to skip electron-hole capture processes occur through TiO 2 surface states to avoid the interfacial recombination. Further increment in the concentration of MoS 2 suppresses the resulting DSSC performance by blocking the porosity which results in less dye adsorption and hence lower photocurrent values.

Thermal energy plays a crucial role on the phase evolution of niobium oxide (Nb 2 O 5 ) thin films and when employed as a blocking layer these films can manoeuvre charge transfer process in a dye sensitized solar cell (DSSC). Niobium oxide film, prepared by RF magnetron sputtering process, endured phase transitions successively from amorphous to orthorhombic and finally to monoclinic phases when subjected to post-deposition annealing. The co-existence of orthorhombic and monoclinic phases with an interesting surface morphology is perceived at an annealing temperature of 900 °C. Nb 2 O 5 blocking layer at the FTO/TiO 2 interface strongly influenced the photovoltaic parameters of the DSSC and the blocking layer in the orthorhombic phase is found to be most effective in suppressing charge recombination and delivered a maximum efficiency of 7.33%. The improvement in open circuit voltage can be foreseeable as shifting of the Fermi level towards the conduction band edge of the TiO 2 as a result of structural modification of the Nb 2 O 5 blocking layer. The thermal stability of the FTO is also investigated and found that the electrical and optical properties of FTO were remarkably stable up to 600 °C and begin to change appreciably from 700 °C onwards.

Tungsten disulphide (WS2) quantum dots, coated on titanium dioxide (TiO2) by chemical bath deposition, increased the photovoltaic performance of dye sensitized solar cells (DSSCs) by ~ 11%. WS2 quantum dots exhibited a dominant optical absorption in a spectral range of 350–800 nm with its characteristics peaks at 550 nm and 620 nm corresponding to the direct bandgap optical transition at the K-point. X-ray photoelectron spectroscopic measurement was performed to study W 4f and S 2p peaks assured the presence of phase-pure WS2 quantum dots. Further, transmission electron microscopic studies showed a distribution of 8.5–20 nm quantum dots with d-spacing of 0.33 nm confirming the WS2. Photovoltaic characteristics of DSSCs with WS2 quantum dots asserted the additional photo-electrons generated by WS2 via the increment in photocurrent density and overall performance. This study demonstrated the possibility of integrating 2D-layered quantum dots into DSSCs as an additional photo-absorbing material candidate along with organic photo-sensitizer such as Ruthenium based N719.

The present work examined dye sensitized 2D layered tungsten disulfide (WS

Photovoltaic performance in dye sensitized solar cells (DSSCs) was improved by incorporating hydrothermal processed molybdenum disulfide (MoS2) into the bulk of titanium dioxide (TiO2) nanoparticle film. MoS2 exhibits a heterogeneous morphology comprising randomly distributed clustered nanoparticles and one dimensional nano-needles. The heterogeneous MoS2 was examined by X-ray photoelectron spectroscopy to study Mo 3d and S 2p peaks. Transmission electron microscopic studies on the heterogeneous MoS2 assert the presence of multilayers which further confirmed by UV–visible optical absorption spectroscopy showed absence of band-edge excitonic peaks at 612 nm and 674 nm. DSSCs show 17% enhancement in performance for 0.09 wt% of heterogeneous MoS2 incorporated TiO2 nanoparticle film compared to reference DSSC fabricated using only TiO2. Further changes in performance was examined by varying the concentration of MoS2 in TiO2 and observed that there is an optimum value to facilitate photo-generated charge transport kinetics in TiO2. The heterogeneous nature of MoS2 effectively acquired photo-electrons from TiO2 due to the presence of conduction band edge few meV below than that of in TiO2 and helps improving the performance.

Major loss factors for photo-generated electrons due to the presence of surface defects in titanium dioxide (TiO2) were controlled by RF-sputtered tungsten trioxide (WO3) passivation. X-ray photoelectron spectroscopy assured the coating of WO3 on the TiO2 nanoparticle layer by showing Ti 2p, W 4f and O 1s characteristic peaks and were further confirmed by X-ray diffraction studies. The coating of WO3 on the TiO2 nanoparticle layer did not affect dye adsorption significantly. Dye sensitized solar cells (DSSCs) fabricated using WO3-coated TiO2 showed an enhancement of ~10% compared to DSSCs fabricated using pristine TiO2-based photo-electrodes. It is attributed to the WO3 passivation on TiO2 that creates an energy barrier which favored photo-electron injection by tunneling but blocked reverse electron recombination pathways towards holes available in highest occupied molecular orbital of the dye molecules. It was further evidenced that there is an optimum thickness (duration of coating) of WO3 to improve the DSSC performance and longer duration of WO3 suppressed photo-electron injection from dye to TiO2 as inferred from the detrimental effect in short circuit current density values. RF-sputtering yields pinhole-free, highly uniform and conformal coating of WO3 onto any area of interest, which can be considered for an effective surface passivation for nanostructured photovoltaic devices.

A physical vapor deposition based molybdenum trioxide (MoO3) thin film is demonstrated as an efficient reverse-electron recombination barrier layer (RBL) at the fluorine doped tin oxide (FTO)/titanium dioxide (TiO2) interface in dye sensitized solar cells (DSSCs). Thin films of MoO3 show an average optical transmittance of ∼77% in a spectral range of 350–800 nm with bandgap value of ∼3.1 eV. For an optimum thickness of MoO3, deposited for 5 minutes, the resulting DSSCs showed 15% enhancement in efficiency (η) compared to the reference DSSC which did not use MoO3 RBL; this suggests that MoO3 is effectively suppressing interfacial recombination at the FTO/TiO2 interface. Further, increasing the thickness of MoO3 RBL at the FTO/TiO2 interface (20 minutes deposition) is observed to impede charge transport, as noticed with 55% reduction in η compared to the reference DSSC. Thin film MoO3 RBL with an optimum thickness value at the FTO/TiO2 interface efficiently blocks the leaky transport pathways in the mesoporous TiO2 nanoparticle layer and facilitates efficient charge transport as confirmed by electrochemical impedance spectroscopy.

Self-assembled heterostructure of 2D-layered tungsten disulfide (WS2) with Cadmium telluride (CdTe) quantum dots is reported. CdTe quantum dots (8 nm–10 nm) were intercalated into 2D-layered WS2 which showed an approximate lateral length distribution of 1 µm. X-ray photoelectron spectroscopic studies confirmed Cd (3d5/2, 3d3/2), Te (3d5/2, 3d3/2), W (4f7/2, 4f5/2) and S (2p3/2, 2p1/2) in the WS2-CdTe heterostructure which exhibited an optical absorption in the spectral range of 200 nm–550 nm. The dominant optical absorption around 250 nm is attributed to the quantum size effects occurred on both WS2 and CdTe. A sharp photoluminescence emission at 600 nm was further observed to be tunable with respect to concentration of CdTe quantum dots into the WS2 layers.

Adding 6 wt% of graphene oxide (GO) into iodide-triiodide (I−/I−3) electrolyte improved dye sensitized solar cell (DSSC) performance by a factor of 100% (from 3.5% to 7.0%) by achieving effective hole transport. Performance of DSSCs was suppressed by a factor of 17% (from 3.5% to 2.9%) due to the GO more than 6 wt%. GO improved the hole transport for an optimum concentration in the I−/I−3 while impeded the same for higher concentration due to the reduction in diffusivity. The presented work established a simple and straight forward approach to improve the performance of DSSCs by improving hole transport.

Microwave treatment in 2D-layered molybdenum diselenide (MoSe2) nanoflakes effectively changed structural, optical and charge transport characteristics at the nanoscale level due to the formation of MoO3-MoSe2 dual-phase. The co-existing MoO3-MoSe2 dual-phase, enabled by microwave treatment in pristine MoSe2, was highly sensitive to microwave thermal energy which caused significant shifts in Raman characteristic peaks due to the formation of dual phase which further confirmed in X-ray diffraction. It was also asserted by X-ray photoelectron spectroscopic studies which showed the effects of microwave induced changes on surface chemical characteristics of pristine MoSe2. Specifically, the loosely packed van der Waals sheet-like layers in pristine MoSe2 were engineered by microwave treatment and resulted in platelet-like nanostructures as evidenced in scanning electron microscopic studies. Further, charge transport characteristics in MoSe2 films were examined under dark, AM1.5, blue and green laser illuminations. Microwave treated MoSe2 showed a greater enhancement in photo-response than pristine MoSe2 due to the co-existing MoO3-MoSe2 dual-phase. Thus, microwave treatment can be considered for the development of 2D-optoelectronics as a feasible method to tune the physical properties according to requirements.

The present study elucidates dispersive electron transport mediated by surface states in tin oxide (SnO

Scalable syntheses of two-dimensional topological insulators are critical to material exploration. We demonstrate a controlled assembly of a two-dimensional V–VI group compound, Sb2Te3 nanoplates (NPs), through a vapor–solid growth process. The physical thickness of Sb2Te3 NPs can be rationally controlled in a wide range, from hundreds of nm down to sub-10 nm. Single-quintuple-layer Sb2Te3 NPs were obtained, with a high domain density of ∼2.465 × 108 cm−2 over a large surface area (1 cm × 1 cm) of a SiO2/Si substrate, verifying a scalable synthesis method. Extensive material analyses were conducted to explore the basic properties of Sb2Te3 NPs using SEM and AFM, etc. HRTEM analysis confirms that the NP samples exhibit a highly crystalline structure and XPS analysis confirms the chemical composition and material stoichiometry. The growth of 2D topological insulator nanostructures may open up new opportunities in surface-state studies and potential applications in low-dissipative electronic systems.

The loss of local dielectric integrity in ultrathin Al 2 O 3 films grown by atomic layer deposition is investigated using conducting atomic force microscopy. I – V spectra acquired at different regions of the samples by constant and ramping voltage stress are analyzed for their pre- and post-breakdown signatures. Based on these observations, the thickness dependent dielectric reliability and failure mechanism are discussed. Our results show that remarkable enhancement in breakdown electric field as high as 130 MV/cm is observed for ultrathin films of thickness less than 1 nm.

Wide-bandgap multilayer oxygenated graphene (MLOG) is used as the recombination barrier layer (RBL) in dye-sensitized solar cell (DSSC). The MLOG is derived from multilayer graphene synthesized by chemical vapor deposition (CVD) and oxygenated with high density/low power oxygen plasma. We observe 19% improvement in photoconversion efficiency of the DSSC with MLOG RBL, as compared with DSSC without RBL. Lifetime of photo-injected electrons at the conduction band of TiO 2 nanoparticle electrode is increased due to the presence of MLOG. This is attributed to effective collection of electrons at the transparent conductor electrode by suppressing interfacial-recombination via TiO 2 surface states. The control over dark current of the DSSC is also significantly enhanced with MLOG as compared with that without RBL as the direct result of improved interfacial quality of the TiO 2 /dye/electrolyte structure.

We demonstrate 2D-layered semiconductor tungsten disulfide (WS 2 ) nanoflakes as an efficient photoactive material in bulk heterojunction (BHJ) solar cells. WS 2 nanoflakes-coated titanium dioxide (TiO 2 ) nanoparticles layer serves as the electron acceptor with poly(3-hexylthiophene) (P3HT) as the hole conductor. The UV–visible spectroscopic measurement yields an optical bandgap of ∼1.7 eV for the WS 2 nanoflakes, confirming the presence of mixed monolayer/few-layers stack. Raman spectra of the WS 2 -coated TiO 2 show a dominant peak at 140 cm −1 for anatase TiO 2 , whereas peaks at 350 cm −1 (in-plane vibration mode) and 420 cm −1 (out-of-plane vibration mode) denote the signature of WS 2 nanoflakes. Photovoltaic performance, electron transport, and recombination at TiO 2 /WS 2 /P3HT interfaces are explored through trap-assisted Shockley–Read–Hall model to elucidate WS 2 as a potential material candidate for efficient solar energy harvesting.

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into liquid electrolyte. An atomic layer deposited (ALD) hafnium oxide (HfO2) ultra thin compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the DSSC performance were studied with dark and illuminated current–voltage and electrochemical impedance spectroscopy measurements. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% compared to a DSSC with a conventional sol–gel processed TiO2 compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, better electronic quality of the compact layer material and the higher compactness, shown by atomic force microscopic images, obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/compact layer/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO2 compact layer.

Dual-layer graphene (DLG) interconnects with hexagonal boron nitride ( h -BN) as intercalated insulating layer have been demonstrated. The DLG employs graphene grown by chemical vapor deposition process with h -BN serving as a barrier preventing interlayer scattering, which degrades carrier transport in multilayer graphene. The conductive behavior in dual-layer structures is compared with monolayer graphene and randomly stacked bilayer graphene. Reduced resistance is observed in DLG, which exhibits higher current-carrying capacity and maximum power density. In addition, DLG wire is shown to be robust under constant voltage stressing (10 V) at an elevated temperature (150 °C) with the mean time to failure $\sim 75$ times higher than that of stacked bilayer graphene wires.

Potential two-dimensional (2D) van der Waals crystals with mechanical flexibility, transparency, and low cost are viable material platforms for future nanodevices. Resistive switching behavior in 2D layered Sb2Te3 nanosheets is demonstrated. Nearly three orders of magnitude switch in sheet resistance were realized for more than 20 cycles. The observed hysteretic behavior is due to the change between crystalline and amorphous phases under a melt-quench-recrystallization mechanism. More importantly, the energy required to amorphize the nanosheets decreases exponentially with layer thickness reduction. It is expected that scaling to the ultimate two-dimensional limit in chalcogenide nanosheet-based phase change devices may meet or even exceed the energy efficiency of neurobiological architectures.

The present study elucidates dispersive electron transport mediated by surface states in tin oxide (SnO2) nanoparticle-based dye sensitized solar cells (DSSCs). Transmission electron microscopic studies on SnO2 show a distribution of ∼10 nm particles exhibiting (111) crystal planes with inter-planar spacing of 0.28 nm. The dispersive transport, experienced by photo-generated charge carriers in the bulk of SnO2, is observed to be imposed by trapping and de-trapping processes via SnO2 surface states present close to the band edge. The DSSC exhibits 50% difference in performance observed between the forward (4%) and reverse (6%) scans due to the dispersive transport characteristics of the charge carriers in the bulk of the SnO2. The photo-generated charge carriers are captured and released by the SnO2 surface states that are close to the conduction band-edge resulting in a very significant variation; this is confirmed by the hysteresis observed in the forward and reverse scan current-voltage measurements under AM1.5 illumination. The hysteresis behavior assures that the charge carriers are accumulated in the bulk of electron acceptor due to the trapping, and released by de-trapping mediated by surface states observed during the forward and reverse scan measurements.

The objective of this work was to investigate the improvement in performance of dye sensitized solar cells (DSSCs) by depositing ultra thin metal oxides (hafnium oxide (HfO 2 ) and aluminum oxide (Al 2 O 3 )) on mesoporous TiO 2 photoelectrode using atomic layer deposition (ALD) method. Different thicknesses of HfO 2 and Al 2 O 3 layers (5, 10 and 20 ALD cycles) were deposited on the mesoporous TiO 2 surface prior to dye loading process used for fabrication of DSSCs. It was observed that the ALD deposition of ultrathin oxides significantly improved the performance of DSSCs and that the improvement in the DSSC performance depends on the thickness of the deposited HfO 2 and Al 2 O 3 films. Compared to a reference DSSC the incorporation of a HfO 2 layer resulted in 69% improvement (from 4.2 to 7.1%) in the efficiency of the cell and incorporation of Al 2 O 3 (20 cycles) resulted in 19% improvement (from 4.2 to 5.0%) in the efficiency of the cell. These results suggest that ultrathin metal oxide layers affect the density and the distribution of interface states at the TiO 2 /organic dye and TiO 2 /liquid electrolyte interfaces and hence can be utilized to treat these interfaces in DSSCs.

We demonstrate a heterojunction solar cell employing multilayer graphene oxide (ML-GO) as the hole-conducting layer and cadmium selenide (CdSe) quantum-dot-coated titanium dioxide (TiO2) as the photoactive material. Multilayer graphene, synthesized by a chemical-vapor-deposition technique, is treated with low-power (25-W) oxygen plasma (~100 sccm) to convert into ML-GO, as confirmed by Raman spectroscopy. The ML-GO exhibits an optical bandgap of 3.6 eV extracted from UV-visible absorption spectroscopy. The heterojunction solar cell is implemented with a stacked heterostructure of ITO/TiO2/CdSe/ML-GO/Pt, yielding a short-circuit current density of 11.2 mA/cm2, an open-circuit voltage of 652 mV, and a photoconversion efficiency of 4.1% under a standard AM 1.5 illumination condition.

Schottky barriers formed by graphene (monolayer, bilayer, and multilayer) on 2D layered semiconductor tungsten disulfide (WS2) nanosheets are explored for solar energy harvesting. The characteristics of the graphene–WS2 Schottky junction vary significantly with the number of graphene layers on WS2, resulting in differences in solar cell performance. Compared with monolayer or stacked bilayer graphene, multilayer graphene helps in achieving improved solar cell performance due to superior electrical conductivity. The all-layered-material Schottky barrier solar cell employing WS2 as a photoactive semiconductor exhibits efficient photon absorption in the visible spectral range, yielding 3.3% photoelectric conversion efficiency with multilayer graphene as the Schottky contact. Carrier transport at the graphene/WS2 interface and the interfacial recombination process in the Schottky barrier solar cells are examined.

We demonstrate extraordinary photoconductive behavior in two-dimensional (2D) crystalline indium selenide (In2Se3) nanosheets. Photocurrent measurements reveal that semiconducting In2Se3 nanosheets have an extremely high response to visible light, exhibiting a photoresponsivity of 3.95 × 10(2) A·W(-1) at 300 nm with an external quantum efficiency greater than 1.63 × 10(5) % at 5 V bias. The key figures-of-merit exceed that of graphene and other 2D material-based photodetectors reported to date. In addition, the photodetector has a fast response time of 1.8 × 10(-2) s and a specific detectivity of 2.26 × 10(12) Jones. The photoconductive response of α-In2Se3 nanosheets extends into ultraviolet, visible, and near-infrared spectral regions. The high photocurrent response is attributed to the direct band gap (EG = 1.3 eV) of In2Se3 combined with a large surface-area-to-volume ratio and a self-terminated/native-oxide-free surface, which help to reduce carrier recombination while keeping fast response, allowing for real-time detection under very low-light conditions.

A two-dimensional layered insulator, hexagonal boron nitride (h-BN), is demonstrated as a new class of surface passivation materials in dye-sensitized solar cells (DSSCs) to reduce interfacial carrier recombination. We observe ∼57% enhancement in the photo-conversion efficiency of the DSSC utilizing h-BN coated semiconductor TiO2 as compared with the device without surface passivation. The h-BN coated TiO2 is characterized by Raman spectroscopy to confirm the presence of highly crystalline, mixed monolayer/few-layer h-BN nanoflakes on the surface of TiO2. The passivation helps to minimize electron–hole recombination at the TiO2/dye/electrolyte interfaces. The DSSC with h-BN passivation exhibits significantly lower dark saturation current in the low forward bias region and higher saturation in the high forward bias region, respectively, suggesting that the interface quality is largely improved without impeding carrier transport at the material interface. The experimental results reveal that the emerging 2D layered insulator could be used for effective surface passivation in solar cell applications attributed to desirable material features such as high crystallinity and self-terminated/dangling-bond-free atomic planes as compared with high-k thin-film dielectrics.

We demonstrate Schottky-barrier solar cells employing a stack of layer-structured semiconductor molybdenum disulfide (MoS(2)) nanomembranes, synthesized by the chemical-vapor-deposition method, as the critical photoactive layer. An MoS(2) nanomembrane forms a Schottky-barrier with a metal contact by the layer-transfer process onto an indium tin oxide (ITO) coated glass substrate. Two vibrational modes in MoS(2) nanomembranes, E(1)(2g) (in-plane) and A(1g) (perpendicular-to-plane), were verified by Raman spectroscopy. With a simple stacked structure of ITO-MoS(2)-Au, the fabricated solar cell demonstrates a photo-conversion efficiency of 0.7% for ~110 nm MoS(2) and 1.8% for ~220 nm MoS(2). The improvement is attributed to a substantial increase in photonic absorption. The MoS(2) nanomembrane exhibits efficient photo-absorption in the spectral region of 350-950 nm, as confirmed by the external quantum efficiency. A sizable increase in MoS(2) thickness results in only minor change in Mott-Schottky behavior, indicating that defect density is insensitive to nanomembrane thickness attributed to the dangling-bond-free layered structure.