Subhrajit Mukherjee

Hydrothermal growth of few layer 2H-MoS2 for heterojunction photodetector and visible light induced photocatalytic applications

The present low yielding growth techniques of semiconducting 2H phase molybdenum disulfide (MoS2) hamper its widespread applications. In this article, we report a novel hydrothermal chemical approach to synthesize micron sized few layer 2H-MoS2 on a large scale. Sodium molybdate and ammonium thiocyanate have been used as precursors to obtain template-free 2H-MoS2 in solution. Detailed microscopic and spectroscopic characterizations reveal that the bottom-up synthesized few layer MoS2 flakes are highly crystalline having the hexagonal 2H phase. Photodetector devices comprising a p-type silicon (p-Si)/n-MoS2 heterostructure have been fabricated for the first time using solution processed 2H-MoS2 synthesized by the bottom up approach. The heterojunction diode exhibits a high rectification ratio (>103) with broad band photoresponse over the visible range. Because of the visible light photoresponse, as-synthesized MoS2 along with reduced graphene oxide (MoS2–RGO hybrids) have been utilized to study the potential of this two dimensional (2D) heterostructure for visible light driven photocatalytic Rhodamine B dye degradation. This study demonstrates the potential of solution processed MoS2 for integration with silicon and growth of 2D heterostructures for visible light induced multifunctional applications.

Novel Colloidal MoS2 Quantum Dot Heterojunctions on Silicon Platforms for Multifunctional Optoelectronic Devices.

Silicon compatible wafer scale MoS2 heterojunctions are reported for the first time using colloidal quantum dots. Size dependent direct band gap emission of MoS2 dots are presented at room temperature. The temporal stability and decay dynamics of excited charge carriers in MoS2 quantum dots have been studied using time correlated single photon counting spectroscopy technique. Fabricated n-MoS2/p-Si 0D/3D heterojunctions exhibiting excellent rectification behavior have been studied for light emission in the forward bias and photodetection in the reverse bias. The electroluminescences with white light emission spectra in the range of 450–800 nm are found to be stable in the temperature range of 10–350 K. Size dependent spectral responsivity and detectivity of the heterojunction devices have been studied. The peak responsivity and detectivity of the fabricated heterojunction detector are estimated to be ~0.85 A/W and ~8 × 1011 Jones, respectively at an applied bias of −2 V for MoS2 QDs of 2 nm mean diameter. The above values are found to be superior to the reported results on large area photodetector devices fabricated using two dimensional materials.

Enhancement of Efficiency of a Solar Cell Fabricated on Black Si Made by Inductively Coupled Plasma–Reactive Ion Etching Process: A Case Study of a n-CdS/p-Si Heterojunction Cell

We show that a significant enhancement of solar cell efficiency can be achieved in cells fabricated on black Si made using inductively coupled plasma-reactive ion etching (ICP-RIE). The ICP-RIE-fabricated black Si results in an array of vertically oriented defect-free Si nanocones (average height ∼150 nm; apex diameter ∼25 nm) exhibiting an average reflectance ≤2% over most of the relevant solar spectral range. The enabling role of the ultralow reflectance of the nanostructured black Si has been demonstrated using a heterojunction solar cell fabricated by depositing a n-type CdS film on p-Si nanocones followed by a transparent conducting coating of Al-doped ZnO (AZO). The fabricated n-CdS/p-Si heterojunction exhibits promising power conversion efficiency close to 3%, up from a mere efficient 0.15% for a similar cell fabricated on a planar Si. The effect of the fabrication process for the black Si on solar cell performance has been investigated through the measurements of carrier lifetime and surface recombination velocity. The accompanying model and simulation analysis shows that the conical structure leads to the effective dielectric constant varying smoothly from the value of the air at the top to the value of Si at the base over the length of the nanocone, leading to a substantial reduction of its reflectance.

Growth of Au capped GeO2 nanowires for visible-light photodetection

A single step process to grow Au capped oxygen deficient GeO2 crystalline nanowires via generation of growth species through the metal induced surface decomposition of Ge substrate is reported. Without the external source supply, the growth of the Au-GeO2 nanowires on the Ge substrate is addressed with possible mechanism. Despite high band gap, application of GeO2 as a possible new material for visible light photodetection is presented. The as-grown samples were found to have a photo-response of ≥102 with 17% external quantum efficiency at −2.0 V applied bias upon visible-light illumination (λ = 540 nm, 0.2 mW/cm2). This visible-light detection can be attributed to the oxygen vacancy related defect states as well as localized surface plasmon resonance induced absorption and subsequent hot electron injection from Au to conduction band of GeO2. The photodetection performance of the devices has been understood by the proposed energy band diagrams. In addition, ≈4 times enhancement in the efficiency has been ...

Studies on carbon dots embedded Tamm plasmon polariton structures

Tamm plasmon polaritons (TPPs) are excellent candidates for photonic device application for their intriguing properties and simple fabrication design. In this study, optical Tamm structures are fabricated by depositing silver thin film on onedimensional photonic crystals (1DPhCs), constituting twelve pairs of alternating quarter wave thick SiO2 and TiO2 thin films. Carbon quantum dots (CQDs) were incorporated in the TiO2 matrix of the final four pairs of the 1DPhCs. TPPs are observed in the reflectance spectra of the samples with and without CQDs. With the help of transfer matrix method electric field intensity distribution profile is obtained. It is observed that the electric field is confined and enhanced at the metal-1DPhC interface and decays within the 1DPhC. Comparison of PL emission from samples with and without CQDs in the last four layers are presented. Enhanced PL emission from CQDs corresponding to the TPP mode and suppression of emission within the photonic stop band is demonstrated.

Effect of size confinement in C exciton dynamics of few-layered MoS 2 nano-sheets

We study transient absorption response of few-layered MoS2 nano-flakes in dispersion, mainly focusing on its high energy exciton (commonly known as C exciton). We use a simple sono-chemical exfoliation technique to obtain confined nano-crystals of MoS2 of average diameter 2 nm, inter-dispersed in the flakes and study the effect of quantum confinement on this layered semiconductor. We emphasize on the interplay between exciton bleaching and excited state absorption upon a blue-detuned pumping. The relaxation times for the exciton are found and for the nano-crystals the radiative relaxation process is found to be slower as compared to that of the nano-flakes.

Single Silicon Nanowire based Broadband Photodetector with Superior Responsivity

Single-silicon-nanowire based MSM photodetectors show superior response (>10000 A/W) even without external bias in visible-to-near-IR region. The responsivity improvement as function of nanowire diameter has been explained by electric-field enhancement using finite-element based optical simulation.

Multi-colour OPO based on second order cascaded nonlinear interaction

Intra-cavity second-order cascaded nonlinear interaction is utilized in a BBO based OPO for generation of multicolour radiation simultaneously. Theoretical investigation extracts the induced effective third order susceptibility and explains the experimental observations.

Investigation on stability of cascaded second order mode-locking

A complete theoretical investigation on an end pumped cw Nd:YVO4 laser mode-locked using soft-aperturing and cascading of second order nonlinearities in an intracavity KTP crystal are reported and also verified with previously done experiment.