Shawn Sanctis

Genetically improved monolayer-forming tobacco mosaic viruses to generate nanostructured semiconducting bio/inorganic hybrids.

The genetically determined design of structured functional bio/inorganic materials was investigated by applying a convective assembly approach. Wildtype tobacco mosaic virus (wt TMV) as well as several TMV mutants were organized on substrates over macroscopic-length scales. Depending on the virus type, the self-organization behavior showed pronounced differences in the surface arrangement under the same convective assembly conditions. Additionally, under varying assembly parameters, the virus particles generated structures encompassing morphologies emerging from single micrometer long fibers aligned parallel to the triple-contact line through disordered but dense films to smooth and uniform monolayers. Monolayers with diverse packing densities were used as templates to form TMV/ZnO hybrid materials. The semiconducting properties can be directly designed and tuned by the variation of the template architecture which are reflected in the transistor performance.

Zinc diketonates as single source precursors for ZnO nanoparticles: microwave-assisted synthesis, electrophoretic deposition and field-effect transistor device properties

The microwave-assisted decomposition of zinc diketonates in acetonitrile leads to stable dispersions of zinc oxide nanoparticles. The variation of the diketonato ligand framework permits controlling the size of primary crystallites and soft agglomerates, which allows the synthesis of nanoparticles in the range of 4–6 nm. Field-effect transistors are fabricated with charge carrier mobility as high as 0.32 cm2 V−1 s−1 and an Ion/off ratio of ∼106 after post-annealing at only 250 °C in air. Their superior performance is attributed to the dense packing of the ZnO particles in the semiconducting layer. Dispersions in aprotic solvent are suitable for a cathodic electrophoretic deposition of ZnO layers on ITO coated glass electrodes. Uniform ZnO coatings exhibiting interference colours can be obtained with thicknesses of several hundred nanometers.

Understanding the temperature-dependent evolution of solution processed metal oxide transistor characteristics based on molecular precursor derived amorphous indium zinc oxide

Amorphous indium zinc oxide (IZO) thin films are accessible by solution-deposition of mixtures of molecular single-source precursors with dimethyl 2-hydroxyimino- and 2-nitromalonato ligands (dmm-NOH and Hdmm-NO2, respectively). Thermal combustion of the precursor molecules In3O3(dmm-NO2)3·(toluene) and [Zn4O(dmm-NO)6] leads to a highly exothermic decomposition reaction yielding amorphous indium zinc oxide (IZO) even at a temperature of 150 °C. The main aim of the present investigation is to correlate the electronic performance in such solution processed field-effect transistors (FET) with the presence of surface groups and bulk defects depending on the processing temperatures of the resulting IZO films (250 to 400 °C). In depth electronic characterization using X-Ray- and Photoelectron Emission Spectroscopy (XPS and UPS) reveals major electronic changes during thin film formation in the temperature range between 275 and 300 °C. These findings are confirmed by Positron Annihilation Spectroscopy (PAS) which allows the monitoring of defects in a picometer range in the resulting functional IZO thin films. Resulting transistor mobilities (μ) of the semiconducting IZO films are in the range of those of amorphous silicon even at a processing temperature of 250 °C and increase up to 6 and 9.5 cm2 (V s)−1 at 350 and 400 °C with on/off ratios of 105 up to 107, respectively.

Molecular Precursors for ZnO Nanoparticles: Field-Assisted Synthesis, Electrophoretic Deposition, and Field-Effect Transistor Device Performance

Zinc complexes with multidentate Schiff base ligands are suitable precursors for ZnO in microwave-assisted transformation reactions. [Bis(acetylacetonato)ethylenediimine]zinc(II) and [bis(methylacetoacetato)ethylenediimine]zinc(II) have been synthesized with high purity and good yield from the direct reaction of the respective diimine ligand with diethylzinc in tetrahydrofuran. The thermal decay is studied by thermogravimetry coupled with online infrared spectroscopy. The ceramization reaction in ethoxyethanol yields stable dispersions of spherical ZnO nanoparticles with very small particle sizes (around 5-6 nm), which can be employed for coating and thin-film deposition processes. Field-effect transistors (FETs) composed of thin films fabricated from these semiconducting ZnO particles possess charge-carrier mobilities of 6.0 × 10-3 and 5.4 × 10-2 cm2/(V s) after processing at 350 and 450 °C, respectively. Electrophoretic deposition affords dense film coatings composed of these ZnO nanoparticles with thicknesses of 30-90 nm on ITO (indium tin oxide) glass-electrodes. The positive ζ-potentials of the ZnO nanoparticles in these dispersions are in agreement with the electrocoating process at the cathode.

Toward an Understanding of Thin-Film Transistor Performance in Solution-Processed Amorphous Zinc Tin Oxide (ZTO) Thin Films

Amorphous zinc tin oxide (ZTO) thin films are accessible by a molecular precursor approach using mononuclear zinc(II) and tin(II) compounds with methoxyiminopropionic acid ligands. Solution processing of two precursor solutions containing a mixture of zinc and tin(II)-methoxyiminopropinato complexes results in the formation of smooth homogeneous thin films, which upon calcination are converted into the desired semiconducting amorphous ZTO thin films. ZTO films integrated within a field-effect transistor (FET) device exhibit an active semiconducting behavior in the temperature range between 250 and 400 °C, giving an increased performance, with mobility values between μ = 0.03 and 5.5 cm2/V s, with on/off ratios increasing from 105 to 108 when going from 250 to 400 °C. Herein, our main emphasis, however, was on an improved understanding of the material transformation pathway from weak to high performance of the semiconductor in a solution-processed FET as a function of the processing temperature. We have correlated this with the chemical composition and defects states within the microstructure of the obtained ZTO thin film via photoelectron spectroscopy (X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy), Auger electron spectroscopy, electron paramagnetic resonance spectroscopy, atomic force microscopy, and photoluminescence investigations. The critical factor observed for the improved performance within this ZTO material could be attributed to a higher tin concentration, wherein the contributions of point defects arising from the tin oxide within the final amorphous ZTO material play the dominant role in governing the transistor performance.

Microwave assisted synthesis and characterisation of a zinc oxide/tobacco mosaic virus hybrid material. An active hybrid semiconductor in a field-effect transistor device

Summary Tobacco mosaic virus (TMV) has been employed as a robust functional template for the fabrication of a TMV/zinc oxide field effect transistor (FET). A microwave based approach, under mild conditions was employed to synthesize stable zinc oxide (ZnO) nanoparticles, employing a molecular precursor. Insightful studies of the decomposition of the precursor were done using NMR spectroscopy and material characterization of the hybrid material derived from the decomposition was achieved using dynamic light scattering (DLS), transmission electron microscopy (TEM), grazing incidence X-ray diffractometry (GI-XRD) and atomic force microscopy (AFM). TEM and DLS data confirm the formation of crystalline ZnO nanoparticles tethered on top of the virus template. GI-XRD investigations exhibit an orientated nature of the deposited ZnO film along the c-axis. FET devices fabricated using the zinc oxide mineralized virus template material demonstrates an operational transistor performance which was achieved without any high-temperature post-processing steps. Moreover, a further improvement in FET performance was observed by adjusting an optimal layer thickness of the deposited ZnO on top of the TMV. Such a bio-inorganic nanocomposite semiconductor material accessible using a mild and straightforward microwave processing technique could open up new future avenues within the field of bio-electronics.

Microwave synthesis and field effect transistor performance of stable colloidal indium-zinc-oxide nanoparticles

Indium zinc oxide nanoparticles were prepared by microwave-assisted decomposition employing solutions of molecular air stable In and Zn precursors of Schiff base type. Stable colloidal ready to use dispersions with shelf-lives of several months were obtained at comparably low temperature of 140 °C by heating mixtures of indium and zinc complexes with Schiff base oximato ligands in 2-ethoxyethanol. IZO particles prepared therefrom with an In : Zn ratio of 60 : 40 display an average diameter of about 5 nm and appeared amorphous in nature. Thin films of the colloidal particles with a uniform surface coverage and low roughness could be obtained by spin-coating on silicon dioxide substrates. X-ray photoelectron spectroscopy showed that oxide formation had occurred after the microwave reaction, but could be improved further by annealing the films at elevated temperatures. The removal of adherent organic and hydroxy moieties at 450 °C thus led to an excellent semiconducting behaviour of the finally resulting IZO films. Obtained thin film transistors exhibited a n-type enhancement mode performance, with a mobility of 8.7 cm2 V−1 s−1, an Ion/off ratio of 2.8 × 105 and a threshold voltage Vth of +3.3 V.

Stacked indium oxide/zinc oxide heterostructures as semiconductors in thin film transistor devices: a case study using atomic layer deposition

Multi-layer heterostructure oxide semiconductors employing a layer-by-layer deposition of alternating indium oxide and zinc oxide thin films generated via atomic layer deposition (ALD) are investigated for their feasibility into high performance thin film transistors (TFT). The successful deposition of uniform film thickness across the alternating indium oxide and zinc oxide deposition at 200 °C is achieved using trimethyl indium (TMI), diethyl zinc (DEZ) and water as oxidizing agent. The as-prepared polycrystalline material shows a conductive behaviour which upon additional mild annealing between 250–300 °C demonstrates a high TFT device performance. In addition, insights into the dependency of the defect passivation gradient within the multilayer upon thermal annealing of the oxide stack are presented. Studies towards an optimised film thickness result in a high device performance in enhancement mode with a saturation field-effect mobility (μsat.) of 6.5 cm2 V−1 s−1 and an on/off ratio (Ion/off) of 4.6 × 107 using a deliberately large width to length channel ratio (W/L = 500). The TFT performance turned out to be dependent on the position of the individual oxide layers within the stack and the number of heterostructure stacks. These findings on the influence of semiconductor stack formation allow for a better understanding on the formation of the active semiconductor channel and serve towards the applicability of ALD based heterostructure metal oxide semiconductors in next generation electronics.

Charge Transport in Low-Temperature Processed Thin-Film Transistors Based on Indium Oxide/Zinc Oxide Heterostructures

The influence of the composition within multilayered heterostructure oxide semiconductors has a critical impact on the performance of thin-film transistor (TFT) devices. The heterostructures, comprising alternating polycrystalline indium oxide and zinc oxide layers, are fabricated by a facile atomic layer deposition (ALD) process, enabling the tuning of its electrical properties by precisely controlling the thickness of the individual layers. This subsequently results in enhanced TFT performance for the optimized stacked architecture after mild thermal annealing at temperatures as low as 200 °C. Superior transistor characteristics, resulting in an average field-effect mobility (μsat.) of 9.3 cm2 V-1 s-1 ( W/ L = 500), an on/off ratio ( Ion/ Ioff) of 5.3 × 109, and a subthreshold swing of 162 mV dec-1, combined with excellent long-term and bias stress stability are thus demonstrated. Moreover, the inherent semiconducting mechanism in such multilayered heterostructures can be conveniently tuned by controlling the thickness of the individual layers. Herein, devices comprising a higher In2O3/ZnO ratio, based on individual layer thicknesses, are predominantly governed by percolation conduction with temperature-independent charge carrier mobility. Careful adjustment of the individual oxide layer thicknesses in devices composed of stacked layers plays a vital role in the reduction of trap states, both interfacial and bulk, which consequently deteriorates the overall device performance. The findings enable an improved understanding of the correlation between TFT performance and the respective thin-film composition in ALD-based heterostructure oxides.

Metal oxide double layer capacitors by electrophoretic deposition of metal oxides. Fabrication, electrical characterization and defect analysis using positron annihilation spectroscopy

Films consisting of nanocrystalline ZnO were deposited on ITO/glass electrodes using an electrophoretic process. The microwave-assisted thermolysis of zinc alkyl-acetoacetates resulted in the formation of stable dispersions for the electrophoretic deposition procedure. Uniform and smooth coatings could be achieved by starting the electrophoresis at lower voltages first and increasing to higher voltages at later stages of the deposition. The ZnO/ITO double layers were integrated in metal oxide semiconductor (MOS) capacitors by completing the set-up with a spin-coated PMMA dielectric layer and gold contacts. The MOS capacitors showed IV curves with a region of negative differential resistance, indicating charge trapping, both in the ZnO grains and at the ZnO/PMMA interface. Doppler broadening positron annihilation (DB-PAS) and positron annihilation life time spectroscopy (PALS) were employed to characterize the point defects and void space within the deposited ZnO layer which allowed to give insight into the bulk composition of the film composition. PALS revealed the presence of micropores in the range of 0.5 to 1.5 nm.