Subho Dasgupta

Macroscopic 3D nanographene with dynamically tunable bulk properties.

Polymer-derived, monolithic three-dimensional nanographene (3D-NG) bulk material with tunable properties is produced by a simple and inexpensive approach. The material is mass-producible, and combines chemical inertness and mechanical strength with a hierarchical porous architecture and a graphene-like surface area. This provides an opportunity to control its electron transport and mechanical properties dynamically by means of electrochemical-induced interfacial electric fields.

Inkjet Printed, High Mobility Inorganic-Oxide Field Effect Transistors Processed at Room Temperature

Printed electronics (PE) represents any electronic devices, components or circuits that can be processed using modern-day printing techniques. Field-effect transistors (FETs) and logics are being printed with intended applications requiring simple circuitry on large, flexible (e.g., polymer) substrates for low-cost and disposable electronics. Although organic materials have commonly been chosen for their easy printability and low temperature processability, high quality inorganic oxide-semiconductors are also being considered recently. The intrinsic mobility of the inorganic semiconductors are always by far superior than the organic ones; however, the commonly expressed reservations against the inorganic-based printed electronics are due to major issues, such as high processing temperatures and their incompatibility with solution-processing. Here we show a possibility to circumvent these difficulties and demonstrate a room-temperature processed and inkjet printed inorganic-oxide FET where the transistor channel is composed of an interconnected nanoparticle network and a solid polymer electrolyte serves as the dielectric. Even an extremely conservative estimation of the field-effect mobility of such a device yields a value of 0.8 cm(2)/(V s), which is still exceptionally large for a room temperature processed and printed transistor from inorganic materials.

Electrical resistivity of nanocrystalline Al-doped zinc oxide films as a function of Al content and the degree of its segregation at the grain boundaries

Highly transparent and conducting Al-doped ZnO (AZO) films are prepared via sol-gel method with a broad range of nominal Al-doping. The film porosity and morphology is determined by the rate of temperature ramping during the drying of the gel phase. The minimum resistivity is observed to occur around 1.5–2 at. % Al-doped films, irrespective of the morphology and microstructure. It is found by local chemical analysis that Al tends to segregate at the grain boundaries and above a critical concentration, the segregated Al starts to dominate the electronic transport in nanocrystalline AZO. The optical measurements corroborate these findings showing a systematic increase in carrier density only up to 1.5–2 at. % Al-doping. It is concluded that the presence of the resistivity minimum is not merely determined by a solubility limit but is a result of the interplay between the changing carrier concentration and carrier scattering at the segregated Al.

A General Route toward Complete Room Temperature Processing of Printed and High Performance Oxide Electronics

Critical prerequisites for solution-processed/printed field-effect transistors (FETs) and logics are excellent electrical performance including high charge carrier mobility, reliability, high environmental stability and low/preferably room temperature processing. Oxide semiconductors can often fulfill all the above criteria, sometimes even with better promise than their organic counterparts, except for their high process temperature requirement. The need for high annealing/curing temperatures renders oxide FETs rather incompatible to inexpensive, flexible substrates, which are commonly used for high-throughput and roll-to-roll additive manufacturing techniques, such as printing. To overcome this serious limitation, here we demonstrate an alternative approach that enables completely room-temperature processing of printed oxide FETs with device mobility as large as 12.5 cm(2)/(V s). The key aspect of the present concept is a chemically controlled curing process of the printed nanoparticle ink that provides surprisingly dense thin films and excellent interparticle electrical contacts. In order to demonstrate the versatility of this approach, both n-type (In2O3) and p-type (Cu2O) oxide semiconductor nanoparticle dispersions are prepared to fabricate, inkjet printed and completely room temperature processed, all-oxide complementary metal oxide semiconductor (CMOS) invertors that can display significant signal gain (∼18) at a supply voltage of only 1.5 V.

Intercalation‐Driven Reversible Control of Magnetism in Bulk Ferromagnets

An extension in magnetoelectric effects is proposed to include reversible chemistry-controlled magnetization variations. This ion-intercalation-driven magnetic control can be fully reversible and pertinent to bulk material volumes. The concept is demonstrated for ferromagnetic iron oxide where the intercalated lithium ions cause valence change and partial redistribution of Fe(3+) cations yielding a large and fully reversible change in magnetization at room temperature.

Ink-Jet Printed CMOS Electronics from Oxide Semiconductors

Complementary metal oxide semiconductor (CMOS) technology with high transconductance and signal gain is mandatory for practicable digital/analog logic electronics. However, high performance all-oxide CMOS logics are scarcely reported in the literature; specifically, not at all for solution-processed/printed transistors. As a major step toward solution-processed all-oxide electronics, here it is shown that using a highly efficient electrolyte-gating approach one can obtain printed and low-voltage operated oxide CMOS logics with high signal gain (≈21 at a supply voltage of only 1.5 V) and low static power dissipation.

Electrolyte-Gated, High Mobility Inorganic Oxide Transistors from Printed Metal Halides

Inkjet printed and low voltage (≤1 V) driven field-effect transistors (FETs) are prepared from precursor-made In2O3 as the transistor channel and a composite solid polymer electrolyte (CSPE) as the gate dielectric. Printed halide precursors are annealed at different temperatures (300-500 °C); however, the devices that are heated to 400 °C demonstrate the best electrical performance including field-effect mobility as high as 126 cm(2) V(-1) s(-1) and subthreshold slope (68 mV/dec) close to the theoretical limit. These outstanding device characteristics in combination with ease of fabrication, moderate annealing temperatures and low voltage operation comprise an attractive set of parameters for battery compatible and portable electronics.

A new class of epitaxial porphyrin metal–organic framework thin films with extremely high photocarrier generation efficiency: promising materials for all-solid-state solar cells

We demonstrate the fabrication of a new class of epitaxial porphyrin metal–organic framework thin films whose photophysical properties can be tuned by the introduction of electron-donating diphenylamine (DPA) groups into the porphyrin skeleton. The attachment of DPA groups results in strongly improved absorption characteristics, yielding the highest photocarrier generation efficiency reported so far. DFT calculations identify a shift of the charge localization pattern in the VBM (lowest unoccupied molecular orbital), confirming that the introduction of the DPA groups is the main reason for the shift of the optical absorption spectrum and the improved photocurrent generation.

Electric field induced reversible tuning of resistance of thin gold films

The change in resistance of nanostructured metals with respect to an applied field is believed to be due to a change in carrier concentration and hence a linear variation of resistance with the surface charge is expected. In this article, we propose a different approach to explain the resistance variation based on a change in the effective thickness of the film due to a shift of the electron density profile resulting from the applied surface charge. The change in effective thickness together with its effect on surface scattering of electrons account for the majority of the observed variation in resistance. The thin film geometry with different thicknesses and hence a controlled variation of the surface-to-volume ratio allows a deep quantitative understanding and interpretation of the observed phenomena. The model presented in this work shows that a nominal nonlinear response of the resistance of a metal on electrochemically applied surface charge does not necessarily indicate an onset of a redox reaction.

Temperature tolerance study of high performance electrochemically gated SnO2 nanowire field-effect transistors

Electrochemically gated field-effect transistors are fabricated with single crystalline SnO2 nanowires as a transistor channel. Excellent transistor performance and a very low-voltage operation (≤2 V) have been demonstrated. Thermal stability of the FETs is systematically examined up to 180 °C; while unchanged transistor characteristics are obtained up to 70 °C; short exposure at 110 °C is also found permissible, making such devices compatible to be integrated directly to organic photovoltaics or to various biomedical appliances.