Gvido Bratina

Optically switchable transistor via energy-level phototuning in a bicomponent organic semiconductor

Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside attaining an improved device performance, to confer a multifunctional nature to the employed materials is key for organic-based logic applications. Here we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HTs hole transport. As a proof-of-concept we exploited this blend as a semiconducting material in organic thin-film transistors. The device illumination at defined wavelengths enabled reversible tuning of the diarylethenes electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write-erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.

Photo-induced intramolecular charge transfer in an ambipolar field-effect transistor based on a π-conjugated donor–acceptor dyad

A π-conjugated tetrathiafulvalene-fused perylenediimide (TTF-PDI) molecular dyad is successfully used as a solution-processed active material for light sensitive ambipolar field-effect transistors with balanced hole and electron mobilities. The photo-response of the TTF-PDI dyad resembles its absorption profile. Wavelength-dependent photoconductivity measurements reveal an important photo-response at an energy corresponding to a PDI-localized electronic π–π* transition and also a more moderate effect due to an intramolecular charge transfer from the HOMO localized on the TTF unit to the LUMO localized on the PDI moiety. This work clearly elucidates the interplay between intra- and intermolecular electronic processes in organic devices.

Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated through spectroscopic ellipsometry

In this study, we have examined the effects of transfer residue and sample annealing on the optical properties of chemical vapor deposited graphene, transferred onto a sapphire substrate. The optical absorption of graphene was obtained from point-by-point inversion of spectroscopic ellipsometry measurements in the visible and ultraviolet ranges (250–800 nm). Measured spectra were analyzed by optical models based on the Fresnel coefficient equations. The optical models were supported by correlated Raman, scanning electron microscopy, and atomic force microscopy measurements. The obtained data were phenomenologically described by a Fano model. Our results show that a residue layer left on graphene can significantly increase its optical absorption in the visible range, compared to an annealed sample.

Arsenic cap layer desorption and the formation of GaAs(001)c(4×4) surfaces

GaAs(001)c(4×4) surfaces were obtained in the 380–450 °C temperature range by thermal desorption of As cap layers from substrates prepared by molecular beam epitaxy. Although reflection high‐energy electron diffraction patterns showed little change in the temperature range explored, in situ scanning tunneling microscopy and Auger spectroscopy, complemented by ex situ atomic force microscopy, indicate that in the lower‐temperature range examined up to 11%–12% of the surface may still be occupied by adsorbed As in the form of wires and particles preferentially oriented along 〈100〉 directions.

Microscopic control of ZnSe–GaAs heterojunction band offsets

Abstract ZnSe-GaAs heterostructures were synthesized by molecular beam epitaxy at room temperature on GaAs(1 1 0) substrates cleaved in situ, and at 290–320°C (Zn/Se beam pressure ratio = 1) on epitaxial GaAs(1 0 0) layers grown on GaAs(1 0 0) wafers. The epitaxial structures were characterized in situ by photoemission spectroscopy and reflection high energy electron diffraction. The photoemission-determined valence band offset was ΔEv = 1.10 ± 0.05 eV for ZnSe- GaAs(1 1 0) heterojunctions and ΔEv = 0.78−0.83 ± 0.07 eV for ZnSe-GaAs(1 0 0) heterojunctions. This germanium layers (4–6 monolayers thick) fabricated at the interface prior to ZnSe deposition yield a decrease in the overall valence band discontinuity for both ZnSe-Ge-GaAs(1 1 0) and ZnSe-Ge-GaAs(1 0 0) heterojunctions.

ZnSe-GaAs heterojunction parameters

Abstract We synthesized ZnSe-GaAs(100) heterostructures by molecular beam epitaxy and characterized in situ the band offsets by means of X-ray photoemission spectroscopy. We measured a valence band offset of 0.78 eV, i.e. 0.3 eV lower than that observed earlier for ZnSe-GaAs(110) heterostructures grown at room temperature. A further reduction in the valence band offset was obtained by fabricating thin pseudomorphic Ge layers in the interface region. The additional Ge-induced interface dipole increases monotically with Ge layer thickness in the 1 to 6 monolayer range, and saturates at higher coverages. The low valence band offset (0.44 eV) observed in the engineered interfaces should improve hole injection in ZnSe-GaAs based n-p laser diodes.

Epitaxial growth and interface parameters of Si layers on GaAs(001) and AlAs(001) substrates

Thin Si epitaxial layers (1–14 monolayers) were fabricated by molecular beam epitaxy on GaAs(001) and AlAs(001) substrates also obtained by molecular beam epitaxy on GaAs(001) wafers. In situ studies by monochromatic x‐ray photoemission show initial layer‐by‐layer Si growth on both substrates with only minor Si indiffusion. Reflection high energy electron diffraction analysis shows good epitaxy with some indication of three‐dimensional growth at Si coverages higher than 4–8 monolayers. Comparison of our results with recent heterojunction theories suggests that the best predictions for the band offsets are obtained with the model solid approach using deformation potentials to describe the effect of strain. The Si epitaxial layers are found to remain stable upon growth of AlAs or GaAs layers on top of the Si layers.

Atomic scale roughness of GaAs(001)2×4 surfaces

The atomic structure and atomic scale roughness of GaAs(001)2×4 surfaces fabricated by molecular beam epitaxy was examined by scanning tunneling and atomic force microscopy. In particular, the size and spatial distribution of atomic steps at the surface was quantitatively determined as a function of annealing time and annealing temperature. Two different parameters are required to fully describe the surface roughness. In general, we found that prolonged annealing under vacuum of surfaces produced by thermal desorption of As cap layers is sufficient to reduce the surface roughness to that typical of as‐grown surfaces.

Single-step solution processing of small-molecule organic semiconductor field-effect transistors at high yield

Here, we report a simple, alternative route towards high-mobility structures of the small-molecular semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene that requires one single processing step without the need for any post-deposition processing. The method relies on careful control of the casting temperature of the semiconductor and allows rapid production of transistors with uniform and reproducible device performance over large areas.