Showing papers by "P. K. Giri published in 2021"

Recent advances in perovskite/2D materials based hybrid photodetectors

Abstract: Since 2009, metal halide perovskites have attracted a great deal of attention in different optoelectronic applications, such as solar cells, photodetectors (PDs), light-emitting diodes, lasers etc, owing to their excellent electrical and optoelectrical properties. However, since the discovery of graphene, atomically thin 2D materials have been the central focus of materials research due to its exciting properties. Thus, integrating 2D materials with perovskite material can be highly promising for various optoelectronic applications, in particular for ultrasensitive photodetection. In these PDs, 2D materials serve various roles, such as charge transport layer, Schottky contacts, photo absorbers, etc, while perovskite is the light-harvesting active layer. In this review, we focus on the recent findings and progress on metal halide perovskite/2D material phototransistors and hybrid PDs. We comprehensively summarize recent efforts and developments of perovskite/graphene, perovskite/transition-metal dichalcogenides, perovskite/black phosphorus, and perovskite/MXene based phototransistor and heterojunction PDs from the perspective of materials science and device physics. The perovskite/2D material phototransistor can exhibit very high photoresponsivity and gain due to the amplification function of transistors and the pronounced photogating effect in 2D material, while perovskite/2D material heterojunction PD can operate without external bias due to built-in potential across the heterojunction. This review also provides state-of-the-art progress on flexible, transparent, self-powered and PD systems and arrays based on perovskite/2D materials. After summarizing the ongoing research and challenges, the future outlook is presented for developing metal halide perovskite/2D material hybrid PDs for practical applications.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory

Abstract: Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi2O2Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi2O2Se/CsPbBr3 heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi2O2Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi2O2Se nanosheet possessing superior electron mobility and CsPbBr3 nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 μs/24 μs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi2O2Se/CsPbBr3 interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼103 A W−1 was achieved in the Bi2O2Se/CsPbBr3 heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi2O2Se layers and additional density of states in Bi2O2Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.

Temperature-dependent Raman studies and thermal conductivity of direct CVD grown non-van der Waals layered Bi2O2Se

Abstract: Layered materials with the van der Waals gap have been extensively studied due to their fascinating properties. However, non-van der Waals type layered Bi2O2Se exhibiting remarkable properties is challenging to grow due to the weak electrostatic interaction among layers. Herein, we present chemical vapor deposition (CVD) growth of an air-stable ultrathin Bi2O2Se semiconductor with high structural and chemical uniformity. By tuning the growth temperature, we obtained ultra-smooth single crystals of few-layer Bi2O2Se (LBOS) on mica and quartz substrates, as confirmed from x-ray diffraction, micro-Raman, and high-resolution TEM analyses. Furthermore, a low-temperature Raman study has been conducted to better realize phonon dispersion in the as-grown LBOS in the temperature range 78–293 K. It is observed that the A1g phonon mode frequency of LBOS varies linearly with the temperature with a first-order temperature coefficient (α) of −0.017 87 ± 0.0011 cm−1 K−1. The broadening of the Raman spectral linewidth with temperature has been explained based on the phonon decay, and a phonon lifetime of 2.08 ps is found for LBOS at absolute zero temperature. Finally, the in-plane thermal conductivity of LBOS is estimated by a non-contact measurement technique in a relatively straightforward way. Taking advantage of the excitation power dependency of the A1g mode and using the first-order temperature coefficient, the in-plane thermal conductivity of LBOS is estimated to be ∼1.6 W/mK. Our results pave the way for large-area CVD growth of LBOS on arbitrary substrates and developing insights into electron–phonon and phonon–phonon interactions in non-van der Waals 2D materials.

Stable deep blue emission with unity quantum yield in organic–inorganic halide perovskite 2D nanosheets doped with cerium and terbium at high concentrations

Abstract: Despite recent efforts, achieving stable deep blue emission with near unity quantum yield from lead-free perovskite has remained a great challenge. In this study, we have developed a novel strategy to achieve stable and deep blue emission with absolute unity photoluminescence (PL) quantum yield (QY) through Ce3+ and Tb3+ doping at high concentrations in 2D CH3NH3PbBr3 nanosheets (NSs) using a solvothermal method. We investigated the role of dopants in achieving the high QY and deep blue emission in a 2D perovskite using density functional theory (DFT) based calculations of its electronic structure. Our studies reveal that with Ce/Tb doping, the thickness of the NSs systematically goes down from 10 layers to bilayers (1.4 nm) at high doping levels and the bandgap of the 2D perovskite layer increases from 2.394 eV to 2.981 eV. The measured bandgap widening with doping is analyzed and explained on the basis of the quantum confinement effect and lattice contraction. Interestingly, by incorporating 70 mol% CeBr3 in the perovskite crystal, we achieved a deep blue emitting nanoplatelet with 100% QY, narrow linewidth (∼24 nm), and a color coordinate of (0.145, 0.054) closely matching with the standard color Rec. 2020 (0.131, 0.046) specification, making it one of the most efficient perovskite blue light emitters reported to date. We also demonstrate much improved storage stability of the Ce and Tb doped NS, fully consistent with the DFT calculations. The low temperature PL study reveals the coexistence of ordered and disordered orthorhombic phases. From DFT calculations, we show that the dopants stabilize the structure with lower formation energy and enrich the conduction band edge states without introducing deep trap states, which is responsible for the high PL QY. The calculation also reveals that Tb doping leads to a substantial increase in the bandgap, which is fully consistent with our experimental results. Finally, the Ce3+ doped CH3NH3PbBr3 blue-emitting nanoplatelet is used as a white light LED with CIE coordinates (0.334, 0.326). This work demonstrates a versatile approach to develop rare-earth doped deep blue-emitting 2D perovskites with exceptionally high PL QY and provides new insights into the structural stability and electronic structure of rare-earth doped 2D perovskites.

Temperature-dependent Raman study and determination of anisotropy ratio and in-plane thermal conductivity of low-temperature CVD-grown PdSe2 using unpolarized laser excitation

Abstract: PdSe2 is a promising two-dimensional noble metal dichalcogenide with inherent in-plane anisotropy due to its pentagonal structure, and it finds applications in electronic, photonic, and thermoelectric devices. Herein, we present the low-temperature (250–290 °C) chemical vapour deposition (CVD) growth of an air-stable atomically thin PdSe2 layer and study its Raman temperature coefficients, anisotropy ratio and thermal conductivity as a function of the layer thickness. Optical microscopy (OM), atomic force microscopy (AFM), and micro-Raman analyses lead to confirmation of the as-grown ultrathin sheet-like bilayer and ribbon-like few-layer PdSe2 on a mica substrate. We found the reciprocal lattice basis vectors |*| = 0.17 nm and |*| = 0.16 nm in single-crystalline PdSe2, acquired via electron diffraction analysis. A low-temperature Raman study has been conducted to understand the phonon dynamics of the as-grown PdSe2 in the temperature range of 83–295 K. This work opens up a new avenue for calculation of the anisotropic ratio without using a polarized laser, which is significant for designing new functional materials. Analysis of the temperature-dependent Raman data revealed the anisotropic ratios to be 1.42 and 1.28 for bilayer and few-layer PdSe2, respectively. Our results affirm that the anisotropic ratio decreases with the increasing layer number. Furthermore, we perform a comparative analysis of the temperature coefficients for transition metal di-chalcogenides (TMDs) and noble transition metal di-chalcogenides (NTMDs) that have been reported so far. For bilayer to few-layer (∼5-layer) CVD-synthesized PdSe2 the temperature coefficients change from −0.006 cm−1 K−1 to −0.024 cm−1 K−1, respectively, which shows a wider variation with thickness compared with conventional TMDs. Via analysis of the mini-planes of PdSe2, we propose the precise vibrational planes responsible for different Raman modes. Broadening of the Raman spectral linewidth with increasing temperature is explained based on the phonon decay process. Finally, the in-plane thermal conductivities of CVD-grown few-layer and bilayer PdSe2 are estimated to be ∼10.1 W m−1 K−1 and ∼36.8 W m−1 K−1, respectively, using a non-contact Raman measurement technique. Our results are significant for the low-temperature CVD growth of bilayer and few-layer PdSe2 on an arbitrary substrate, including mica, and for gaining insight into electron–phonon and phonon–phonon interactions in noble transition metal chalcogenide 2D materials.

Facile synthetic route to exfoliate high quality and super-large lateral size graphene-based sheets and their applications in SERS and CO2 gas sensing

Abstract: A combination of low-cost synthetic route and simplified exfoliation technique to develop high-quality graphene-based sheets with very large lateral dimensions, which are viable to scale up, remains a challenging problem. Herein, super-large graphene oxide (GO) sheets with lateral size up to 104 μm with a surface area of 6831 μm2 have been developed based on a simple approach using mild heating conditions, and subsequent deoxygenation yields reduced graphene oxide (rGO) sheets. With the decrease in number of layers (<10, <5, bi-layer and mono-layer) in GO, the Raman intensity ratio, ID/IG value increases systematically from 0.73 to 0.97. The efficacy of reducing oxygen-containing functional groups from GO to rGO is confirmed from Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-visible absorption spectroscopy, photoluminescence, and thermogravimetric analysis. Current–voltage measurements revealed substantial improvement of current by three orders of magnitude upon reduction of GO to rGO, which is consistent with the significant decrease in charge transfer resistance in rGO, as revealed from the electrochemical impedance spectra. The large-area GO and rGO sheets when applied in surface-enhanced Raman scattering (SERS) exhibited a large enhancement factor of 104 and high detection capability down to a concentration of 10 nM for Rhodamine B. Furthermore, the rGO incorporated hybrid rGO–SnO2 demonstrated ∼50% improvement in sensitivity for CO2 gas sensing as compared to the commercial SnO2 based gas sensor. The higher sensitivity in the rGO case is ascribed to its high surface area, as revealed from the BET analysis. Therefore, the present simplified and economical approach of large-area graphene oxide could potentially open up a new strategy for industrial-scale production in the future.

3D/2D Bi2S3/SnS2 heterostructures: superior charge separation and enhanced solar light-driven photocatalytic performance

Abstract: In promoting the application of green and sustainable solutions towards the photodegradation of organic dyes and toxic ions, it is urgent to fabricate semiconductor-based effective and stable photocatalysts. Constructing a heterojunction is the new way to accelerate the photoinduced charge separation to achieve enhanced photocatalytic activity. In this article, a novel 3D/2D heterostructure has been fabricated by a simple two-step solvothermal process. First of all, 3D Bi2S3 urchins were synthesized and after that 2D SnS2 nanosheets were decorated on Bi2S3 urchins. The formation of Bi2S3 urchins was monitored at different reaction times and probed by scanning electron microscopy. Both theoretical calculations and experiments suggest that an epitaxial relationship was formed between the (211) plane of Bi2S3 and the (012) plane of SnS2. A type-II band alignment between Bi2S3 and SnS2 was established by theoretical investigation, which accelerated photoinduced charge separation and improved the photocatalytic activity of the Bi2S3/SnS2 heterostructure. The surface area analysis of the Bi2S3/SnS2 heterostructure with different SnS2 loading was monitored and the increased surface area and the porous structure make the heterostructures more active than that of the pure one. Therefore, this 3D/2D heterostructure is found to be important as a new generation photocatalytic system.

Exciton-Plasmon Coupling and Giant Photoluminescence Enhancement in Monolayer MoS2 Through Hierarchically Designed TiO2/Au/MoS2 Ternary Core-Shell Heterostructure

Abstract: Enhancing the light coupling efficiency of large-area monolayer molybdenum disulfide (1L-MoS2) is one of the major challenges for its successful applications in optoelectronics and photonics Herein, we demonstrate a dramatically enhanced photoluminescence (PL) emission from direct chemical vapor deposited monolayer MoS2on a fluorine-doped TiO2/Au nanoparticle plasmonic substrate, where the PL intensity is enhanced by nearly three orders of magnitude, highest among the reported values The formation of TiO2/Au/1L-MoS2ternary core-shell heterojunction is evidenced by the high-resolution transmission electron microscopy and Raman analyses Localized surface plasmon resonance induced enhanced absorption and improved light coupling in the system was revealed from the UV-vis absorption and Raman spectroscopy analyzes Our studies reveal that the observed giant PL enhancement in 1L-MoS2results from two major aspects: firstly, the heavy p-doping of the MoS2lattice is caused by the transfer of the excess electrons from the MoS2to TiO2at the interface, which enhances the neutral exciton emissions and restrains the trion formation Secondly, the localized surface plasmon in Au NPs underneath the 1L-MoS2film initiates exciton-plasmon coupling between excitons of the 1L-MoS2and surface plasmons of the Au NPs at the MoS2/Au interface The PL and Raman analyses further confirm the p-doping effect We isolate the contributions of plasmon enhancement from the theoretical calculation of the field enhancement factor using the effective medium approximation of plasmonic heterostructure, which is in excellent agreement with the experimental data This work paves a way for the rational design of the plasmonic heterostructure for the effective improvement in the light emission efficiency of 1L-MoS2, and may enable engineering the different contributions to enhance the optoelectronic performance of 2D heterostructures