Abhijit Bera

p–i–n Heterojunctions with BiFeO3 Perovskite Nanoparticles and p- and n-Type Oxides: Photovoltaic Properties

We formed p-i-n heterojunctions based on a thin film of BiFeO3 nanoparticles. The perovskite acting as an intrinsic semiconductor was sandwiched between a p-type and an n-type oxide semiconductor as hole- and electron-collecting layer, respectively, making the heterojunction act as an all-inorganic oxide p-i-n device. We have characterized the perovskite and carrier collecting materials, such as NiO and MoO3 nanoparticles as p-type materials and ZnO nanoparticles as the n-type material, with scanning tunneling spectroscopy; from the spectrum of the density of states, we could locate the band edges to infer the nature of the active semiconductor materials. The energy level diagram of p-i-n heterojunctions showed that type-II band alignment formed at the p-i and i-n interfaces, favoring carrier separation at both of them. We have compared the photovoltaic properties of the perovskite in p-i-n heterojunctions and also in p-i and i-n junctions. From current-voltage characteristics and impedance spectroscopy, we have observed that two depletion regions were formed at the p-i and i-n interfaces of a p-i-n heterojunction. The two depletion regions operative at p-i-n heterojunctions have yielded better photovoltaic properties as compared to devices having one depletion region in the p-i or the i-n junction. The results evidenced photovoltaic devices based on all-inorganic oxide, nontoxic, and perovskite materials.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy

We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Improvement in PbS-based Hybrid Bulk-Heterojunction Solar Cells through Band Alignment via Bismuth Doping in the Nanocrystals

We introduce dopants in lead sulfide (PbS) quantum dots (QDs) in forming hybrid bulk-heterojunction (BHJ) solar cells. Because an increase in the content of bismuth as dopants in PbS QDs transforms the intrinsic p-type semiconductor into an n-type one, the band alignment between a conjugated polymer and the doped QDs changes upon doping affecting performance of BHJ solar cells. From scanning tunneling spectroscopy (STS) of the doped QDs, we observe a shift in their Fermi energy leading to formation of a type II band alignment in the polymer:doped-QD interface. We also show that the dopants improve electron-conduction process through the QDs. With the dopants controlling both band alignments at the interface and the conduction process, we show that the dopant concentration in QDs influences open-circuit voltage unfavorably and short-circuit current in a beneficial manner. The device performance of hybrid BHJ solar cells is hence maximized at an optimum concentration of bismuth in PbS QDs.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog.

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.

Molecular rectifiers based on donor/acceptor assemblies: effect of orientation of the components' magnetic moments

In forming donor/acceptor assemblies that act as molecular rectifiers, we have introduced magnetic organic molecules as electron-donating and electron-accepting moieties. We have oriented the magnetic moment of the donor and acceptor components separately and immobilized them (and their moments) so that the molecular assemblies that act as rectifiers could be formed with moments mutually parallel or anti-parallel to each other. We have characterized the molecular assemblies formed on an electrode with a scanning tunneling microscope tip. Such donor/acceptor assemblies with a control over the orientation of moments of the components provided unique systems to study the effect of the nature of alignment on molecular rectifiers. We have observed that the rectification ratio increased in junctions with moments of the components being parallel to each other. The improvement in the rectification ratio has been explained in terms of an efficient electron-transfer process in a moment-aligned junction between the donor and acceptor moieties.

Differential conductance (dI/dV) imaging of a heterojunction-nanorod

Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.

Current Rectification through Vertical Heterojunctions between Two Single-Layer Dichalcogenides (WSe2|MoS2 pn-Junctions)

We form junctions between two single layers of p-type WSe2 and n-type MoS2 in both sequences. The WSe2|MoS2 and MoS2|WSe2 junctions of ultimate thickness limit exhibit current rectification when characterized vertically with a scanning tunneling microscope (STM) tip. The direction of rectification in the pn-junction is opposite to that of the np-junction, confirming occurrence of the rectification to be due to the junctions themselves. From scanning tunneling spectroscopy (STS) and correspondingly the density of states (DOS), we locate the conduction and valence band edges (CB and VB, respectively) of the materials inferring their single-layer and 2H phase configuration. Band edges of the semiconductors form a type-II band alignment resulting in current rectification. In junctions of WSe2 and MoS2 with single layers having a partial overlap, we map band edges along different points on individual semiconductors and the overlapped region (junction). The results have provided experimental evidence of current rectification through van der Waals vertical heterojunctions between two single layers.

Aligned magnetic domains in p- and n-type ferromagnetic nanocrystals and in pn-junction nanodiodes.

We form pn- and np-junctions between monolayers of p- and n-type nanocrystals that exhibit current rectification in the nanodiodes when characterized with a scanning tunneling microscope (STM) tip. With the use of ferromagnetic nanocrystals, we study the effect of mutual alignment of magnetization vectors on current rectification in the junction between the two nanocrystals. We show that when the magnetization vectors of the p- and of the n-type nanocrystals are parallel to each other (and both facing toward the apex of the STM tip) tunneling current in both bias modes increases with correspondingly a higher rectification ratio. This is in contrast to the parameters of the nanodiodes in which magnetization vectors of the components are unaligned or randomized. To analyze the results, we record scanning tunneling spectroscopy of the monolayer of the components having magnetization vectors aligned or unaligned to locate their valence and conduction band edges and to determine the effect of the alignment on the band edges. Upon alignment of the magnetization vectors of the nanocrystals in a monolayer, the conduction band edge of the p-type and valence band edge of the n-type semiconductor shift towards the Fermi energy leading to a change in energy levels of the pn-junctions and accounting for the improved parameters of the nanodiodes.

2D molecular precursor for a one-pot synthesis of semiconducting metal sulphide nanocrystals

Abstract2D molecular materials, namely, metal alkyl thiolates, have been used as a single-source precursor for the synthesis of semiconducting metal sulphide nanocrystals (NCs) by thermal decomposition. These 2D molecular precursors have all the ingredients required for metal sulphide synthesis (metal source, sulphur source and protecting ligand). In this study, we demonstrate a simple and general ‘solvothermal decomposition’ approach for the synthesis of high-quality