Electronic Properties Of Thin Films Research Articles

Electronic properties of polyethylene naphthalate as derived from photo-stimulated discharge, luminescence experiments and quantum chemical calculation

The electronic properties of thin films of poly(ethylene 2,6-naphthalate)—PEN, are investigated based on their photo-physical (optical absorption, photoluminescence) and electrical (space charge distribution, photo-stimulated discharge) behavior. Photo-stimulated currents are associated with optical absorption of the material leading to space charge dissipation as demonstrated by space charge distribution measurement. Based on this set of experimental results and quantum chemical calculation performed on PEN macromolecular system, we propose a new scheme for the electronic levels of PEN. This scheme allows understanding the mechanisms at play in photo-stimulated discharge. One of the main conclusions of our work is that photo-stimulated current measurements do not probe the energy level of traps. Detrapping of charges results from a two-step process where the photon energy is absorbed by chromophores that restitute a part of this energy to trapped charges through various mechanisms. Moreover, the new scheme allows discussing the components of the luminescence excited under different stresses, being electric field, electronic and UV irradiation, charge recombination and thermal activation.

Localized electronic vacancy level and its effect on the properties of doped manganites

Oxygen vacancies are common to most metal oxides and usually play a crucial role in determining the properties of the host material. In this work, we perform ab initio calculations to study the influence of vacancies in doped manganites text {La}_{(1-text {x})} text {Sr}_{text {x}} text {MnO}_{3}, varying both the vacancy concentration and the chemical composition within the ferromagnetic-metallic range (0.2,<,text {x},<,0.5). We find that oxygen vacancies give rise to a localized electronic level and analyse the effects that the possible occupation of this defect state can have on the physical properties of the host. In particular, we observe a substantial reduction of the exchange energy that favors spin-flipped configurations (local antiferromagnetism), which correlate with the weakening of the double-exchange interaction, the deterioration of the metallicity, and the degradation of ferromagnetism in reduced samples. In agreement with previous studies, vacancies give rise to a lattice expansion when the defect level is unoccupied. However, our calculations suggest that under low Sr concentrations the defect level can be populated, which conversely results in a local reduction of the lattice parameter. Although the exact energy position of this defect level is sensitive to the details of the electronic interactions, we argue that it is not far from the Fermi energy for optimally doped manganites (text {x},sim ,1/3), and thus its occupation could be tuned by controlling the number of available electrons, either with chemical doping or gating. Our results could have important implications for engineering the electronic properties of thin films in oxide compounds.

Coupling Lattice Instabilities Across the Interface in Ultrathin Oxide Heterostructures.

Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO3 on the electronic properties of thin films of SrIrO3, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO3 orthorhombic domains couple directly to tetragonal domains in SrTiO3. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO3 film. The close proximity to the metal–insulator transition in ultrathin SrIrO3 causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure–property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.

Dipolar Substitution Impacts Growth and Electronic Properties of Para‐Sexiphenyl Thin Films

AbstractIt is shown that fluorination of para‐sexiphenyl (6P) at the meta‐ or ortho‐positions of one terminal phenyl ring, as well as the addition of a terminal cyano‐group has an eminent impact on both growth and electronic properties of thin films. X‐ray diffraction techniques indicate that films of meta‐substituted 6P (m‐2F‐6P) develop a smooth, layered structure showing crystalline order within the layers only. Contrary, both ortho‐substituted (o‐2F‐6P) and cyano‐substituted 6P (CN‐6P) form films of 3D crystalline order. The correlation of structural information with data from ultraviolet photoelectron spectroscopy reveals that m‐2F‐6P and CN‐6P do not show preferentially oriented dipoles in the film, while o‐2F‐6P grows with collective downward orientation of the dipole moments. The subtle difference in substitution position has a dramatic impact on the thin‐film ionization energy, which increases from 5.40 (ortho) to 6.80 eV (meta) and reaches even 7.50 eV for CN‐6P. The present study shows that nonsymmetric substitution, which induces molecular dipole moments via intramolecular polar bonds, strongly impacts structure, morphology, and electronic properties of thin films. Thusly modifying common conjugated organic materials represents a valuable tool to establish smooth, crystalline layers with altered energetics at hetero‐interfaces with organic or electrode materials in electronic devices.

Space charge-limited current transport in thin films of alkyl-functionalized silicon nanocrystals

We describe the fabrication and electrical characterization of all-silicon electrode devices to study the electronic properties of thin films of silicon nanocrystals (SiNCs). Planar, highly doped Si electrodes with contact separation of 200 nm were fabricated from silicon-on-insulator substrates, by combination of electron beam lithography and reactive ion etching. The gaps between the electrodes of height 110 nm were filled with thin-films of hexyl functionalized SiNCs (diameter 3 nm) from colloidal dispersions, via a pressure-transducing PDMS (polydimethylsiloxane) membrane. This novel approach allowed the formation of homogeneous SiNC films with precise control of their thickness in the range of 15–90 nm, practically without any voids or cracks. The measured conductance of the highly resistive SiNC films at high bias voltages up to 60 V scaled approximately linearly with gap width (5–50 μm) and gap filling height, with little device-to-device variance. We attribute the observed, pronounced hysteretic current–voltage (I–V) characteristics to space-charge-limited current transport, which—after about twenty cycles—eventually blocks the current almost completely. We propose our all-silicon device scheme and gap filling methodology as a platform to investigate charge transport in novel hybrid materials at the nanoscale, in particular in the high resistivity regime.

Optical and Electronic Analysis of Pure and Fe-Doped ZnO Thin Films Using Spectroscopic Ellipsometry and Kramers–Kronig Method

In the present work, pure and Fe-doped ZnO thin films were deposited on glass substrates by sol–gel method. Zinc acetate and iron nitrate were used as the starting material and dopant source, respectively. The concentration of Fe doping was 6[Formula: see text]at.% and 8[Formula: see text]at.%. The optical and electronic properties of pure and Fe-doped ZnO thin films such as refraction index, extinction coefficient, dielectric function and optical band gap energy of the layers were obtained by spectroscopic ellipsometry method in the wavelength range of 300–900[Formula: see text]nm. The incidence angle of the layers kept 70[Formula: see text]. Also data obtained by Kramers–Kronig relations were used for comparison. The influence of Fe-doping concentration on the optical and electronic properties of thin films was studied. The transmittance data of ZnO thin films showed that 6[Formula: see text]at.% Fe-doped ZnO thin film has the highest transmittance value. Dielectric function of pure ZnO films was found to be higher compared with Fe-doped ones. Also it can be deduced from the results that Fe doping influences the optical band gap energy of thin films.

The optical and electrical properties of F doped ZnO thin film by different post-annealing temperatures

The optical and electronic properties of thin films of FZO (F:ZnO = 2:98 at.%) after annealing at different temperature was investigated. The films used in this study were prepared by Radio Frequency Magnetron Sputtering (RF-sputtering) on Corning glass substrates. The results show that the FZO transparent conductive films made at a sputtering power of 50 W, in an Argon atmosphere 15 sccm, after 90 min of deposition at a pressure of 6 mtorr, annealed at 400 °C had the lowest electrical resistivity, and an average transmittance of 85.16% at 400 nm. XRD analysis revealed a peak of growth in the (002) direction which rose with annealing temperature indicating improved crystallinity.

Creating better superconductors by periodic nanopatterning

The quest to create superconductors with higher transition temperatures is as old as superconductivity itself. One strategy, popular after the realization that (conventional) superconductivity is mediated by phonons, is to chemically combine different elements within the crystalline unit cell to maximize the electron-phonon coupling. This led to the discovery of NbTi and Nb_33Sn, to name just the most technologically relevant examples. Here, we propose a radically different approach to transform a ‘pristine’ material into a better (meta-) superconductor by making use of modern fabrication techniques: designing and engineering the electronic properties of thin films via periodic patterning on the nanoscale. We present a model calculation to explore the key effects of different supercells that could be fabricated using nanofabrication or deliberate lattice mismatch, and demonstrate that specific pattern will enhance the coupling and the transition temperature. We also discuss how numerical methods could predict the correct design parameters to improve superconductivity in materials including Al, NbTi, and MgB_22.

Density functional study of BiSbTeSe2 topological insulator thin films

In this work, using density functional theory calculations, we have investigated the band topology of bulk BiSbTeSe2 and its thin film electronic properties in several thicknesses. It is one member of the quaternary compounds Bi2–xSbxTe3–ySey (BSTS) with the best intrinsic bulk insulating behavior. Based on our calculations we have found that a band inversion at Γ‐point is induced when spin–orbit coupling is turned on, with an energy gap of about 0.318 eV. The film thickness has an effect on the surface states such that a gap opens at Dirac point in 6 quintuple‐layers film and with decrease in thickness, the magnitude of the gap increases. The atomic contributions have been mapped out for the first few layers of thin films to demonstrate the surface states. The relative charge density has been calculated layer‐wise and the penetration depth of the surface states into the bulk region is found to be about 2.5–3.5 quintuple layers, depending on the termination species of thin films.

Electronic Properties of Cu2–xSe Nanocrystal Thin Films Treated with Short Ligand (S2–, SCN–, and Cl–) Solutions

To exploit interesting electronic properties of colloidal semiconductor nanocrystals (NCs) in thin film devices, replacement of the original bulky ligands attached on the NC surface to short ones is essential. Here, we investigate the electronic properties of thin films of Cu2–xSe NCs treated chemically with short sulfide (S2–), thiocyanate (SCN–), and chloride (Cl–) ligands that are known to yield superior physical properties compared to the first-generation short ligand systems including amines and thioles. Specifically, the study focuses on the impact of ligand treatment on their direct/indirect bandgap and NIR-localized surface plasmon resonance (LSPR) in the near-IR regime as well as their electrical conductivity and thermoelectric properties. While the application of S2– solution resulted in exchange of the original oleylamine (OAm) on NC surface with S2– ligands, use of SCN– and Cl– solutions only removed the original ligands. The different ligands consistently led a red-shift of the direct and ind...

Synthesis, annealing, characterization, and electronic properties of thin films of a quaternary semiconductor; copper zinc tin sulfide

ABSTRACTCopper zinc tin sulfide thin films were successfully prepared by spin coating on Molybdenum-coated glass substrates. The as-deposited thin films were annealed at 400°C, 450°C, and 500°C in an Argon atmosphere for 1 hr. X-ray diffraction studies revealed that the thin films present the kësterite phase. The band gap decreased with increasing annealing temperature. Schottky barrier-type structures were fabricated and showed a rectifying behavior. The estimated values of the barrier height, the built-in voltage, and the doping density were, respectively, ΦB = 0.71 eV, Na = 5 × 1018 cm−3, and Vbi = 0.33 V for annealed films.

Intercorrelation of Electronic, Structural, and Morphological Properties in Nanorods of 2,3,9,10-Tetrafluoropentacene.

We evidence the intercorrelation of electronic, structural, and morphological properties in nanorods of a substituted fluorine-based pentacene, 2,3,9,10-tetrafluoropentacene, deposited on gold single crystals by using photoemission and X-ray absorption spectroscopy investigations. Our investigations show changes in the XPS spectroscopy lines, and NEXAFS features correlate with the specific structure of the assemblies and their morphology. Consequently, the chemical structure affects not only the molecular electronic structure and the way the molecules assemble in a film but also the film morphology leading to specific thin film electronic properties.

Thickness dependent structural ordering, degradation and metastability in polysilane thin films: A photoluminescence study on representative σ-conjugated polymers

We present a fundamental experimental study based on the fluorescence investigation of thin σ-conjugated polymer films, where the dependence of optoelectrical properties and UV degradation on film thickness ranging from nano- to microscale was studied. Such extensive and detailed study was performed for the first time and observed spectral shifts in emission and excitation spectra and UV degradation retardation point towards the conclusions that there exists a threshold thickness where the material degradation behavior, electron delocalization and structure suddenly change. The development of well aligned polymeric chain structure between the nano- and micrometer thickness (on the mesoscale) was shown responsible for the manifested phenomena. The material thicker than critical 500nm has extremely small Stokes' shift, maximum extended σ-delocalization along the silicon polymer backbone and exhibits remarkable UV degradation slowdown and self-recovery ability. On the contrary, the electronic properties of thin films below 80nm resemble those of random coils in solutions. The films of moderate thickness show relatively steep transition between these two modes of structural ordering and resulting properties. Altogether, we consider this complex phenomenon as a consequence of the mesoscale effect, which is an only recently introduced concept in polymer thin films.

Novel interpenetrating Cu–Al2O3 structures by controlled reduction of bulk CuAlO2

Bulk CuAlO2 with the delafossite structure was processed using two different methods. In one case, stoichiometric CuAlO2 powder was processed using the solution technique, and consolidation was carried out by pressureless sintering in air. In the second case, high purity starting powders of alumina and copper (I) oxide were consolidated by spark plasma sintering. The sintered body was then subjected to a high temperature anneal in air. Reduction of the bulk CuAlO2 samples resulted in a novel composite microstructure with copper forming a continuous intergranular phase. The prior CuAlO2 grains decomposed into an intimate two-phase mixture of metallic copper and θ-alumina. The scale of the composite structures ranged from nanometers to microns. Previous work on the delafossite family has focused primarily on the electronic properties of thin films. The present study suggests that the reduction of this class of mixed oxides gives rise to metal–ceramic structures with unique interfacial and morphological properties.

Optical and electronic properties of thin films and solutions of functionalized forms of graphene and related carbon materials

With the advent of many different forms of graphene (in addition to the long standing interest in fullerenes and carbon nanotubes), the characterization of carbon materials and their degree of conjugation is a particularly important topic in carbon science. We report the application of absorption spectroscopy and conductivity measurements to graphene derivatives and related carbon materials. Reduced graphene oxide (RGO) has been widely studied as a promising material for graphene applications because of its processability, and the same considerations apply to graphene functionalized with octadecylamine (ODA). This study utilizes a direct chemical modification of GO with ODA in the presence of dicyclohexylcarbodiimide to partially regenerate the graphene π-conjugation while producing a material (ODA-G) with excellent solubility in organic solvents. We show that the ODA-G exhibits optical absorptions, which are much stronger than those of GO, and comparable to liquid phase exfoliated graphene and reduced graphene oxide; the optical analysis is unified by the development of an analytical relationship between solid state absorption coefficients and solution state extinction coefficients, which allows the direct comparison of the electronic absorption spectroscopy of films and solutions of carbon materials. We further benchmark the regeneration of the graphenic structure in ODA-G with electrical conductivity measurements.

Conjugated Thiophene-Containing Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties

We report a direct and facile synthesis of novel conjugated polymeric zwitterions (CPZs) as a simple route to electronically active homopolymers and copolymers containing dipole-inducing pendent zwitterions. Sulfobetaine-containing polythiophenes (PTSB-1 and PTSB-2) and alternating thiophene–benzothiadiazoles (PTBTSB-1 and PTBTSB-2) were prepared and characterized relative to alkylated polymer analogues (POT-a-T and POT-a-BT). The polar zwitterionic side chains make these polymers hydrophilic and salt-responsive, with interesting electronic properties that depend on zwitterion distance from the conjugated polymer backbone (tether length), as characterized by UV–vis absorption and ultraviolet photoelectron spectroscopy (UPS). Close proximity (CH2 spacer) of the sulfobetaine groups to the polymer backbone results in increased ionization potential and enlarged band gaps of 2.19 and 2.04 eV for PTSB-1 and PTBTSB-1, respectively. On Au and Ag surfaces, the zwitterionic pendent groups significantly alter the work function due to the presence of an interfacial dipole, with the largest interfacial dipoles measuring −1.29 eV (PTBTSB-1/Au) and −0.69 eV (PTBTSB-1/Ag).

Impact of Fluorination on Initial Growth and Stability of Pentacene on Cu(111)

Structure, growth, thermal stability, and electronic properties of thin films of the fully fluorinated analogue to the archetypical organic semiconductor pentacene (PEN), perfluoropentacene (PFP), were investigated on Cu(111) at room temperature by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). In contrast to PEN, where molecules could only be imaged by STM at full monolayer coverage, PFP was seen to stabilize in disordered clusters already in the submonolayer regime. Furthermore, while long-range order was observed for closed PEN molecular monolayers, PFP only formed a disordered first wetting layer. Highly ordered domains were not observed until the formation of the second layer of PFP. In this layer, the molecular planes are inclined to the surface, as supported by additional STM measurements on graphite and theoretical modeling. Careful consideration of the structural details in the transitional growth regime from molecular mono- to multilayers thus emerges as the key ingredient to achieving a deeper understanding of metal/organic interfaces relevant for organic electronic devices.

Thin Film Electronic Properties of Ternary Topological Insulator

ABSTRACTUsing an ab initio density functional theory (DFT), we study thin film electronic properties of topological insulators (TIs) based on ternary compounds of Tl (thallium) and Bi (bismuth). We consider TlBiX2 (X=Se, Te) and Bi2X2Y (X, Y=Se, Te) compounds. Here we discuss the nature of surface states, their locations in the Brillouin Zone (BZ) and their interactions within the bulk region. Our calculations suggest a critical film thickness to maintain the Dirac cone which is smaller than that in binary Bi-based compounds. Atomic relaxations are found to affect the Dirac cone in some of these compounds. We discuss the penetration depth of surface states into the bulk region.

Density functional study of ternary topological insulator thin films

Using an ab-initio density functional theory based electronic structure method with a semi-local density approximation, we study thin-film electronic properties of two topological insulators based on ternary compounds of Tl (Thallium) and Bi (Bismuth). We consider TlBiX$_2$ (X=Se, Te) and Bi$_2$$X$_2$Y (X,Y= Se,Te) compounds which provide better Dirac cones, compared to the model binary compounds Bi$_2$X$_3$ (X=Se, Te). With this property in combination with a structurally perfect bulk crystal, the latter ternary compound has been found to have improved surface electronic transport in recent experiments. In this article, we discuss the nature of surface states, their locations in the Brillouin zone and their interactions within the bulk region. Our calculations suggest a critical thin film thickness to maintain the Dirac cone which is significantly smaller than that in binary Bi-based compounds. Atomic relaxations or rearrangements are found to affect the Dirac cone in some of these compounds. And with the help of layer-projected surface charge densities, we discuss the penetration depth of the surface states into the bulk region. The electronic spectrum of these ternary compounds agrees very well with the available experimental results.

Electronic properties of Co2MnSi thin films studied by hard x-ray photoelectron spectroscopy

This work reports on the electronic properties of thin films of the Heusler compound Co2MnSi studied by means of hard x-ray photoelectron spectroscopy (HAXPES). The results of photoelectron spectroscopy from multilayered thin films excited by photons of 2–8 keV are presented. The measurements were performed on (substrate/buffer layer/Co2MnSi(z)/capping layer) multilayers with a thickness z ranging from 0 to 50 nm. It is shown that high energy spectroscopy is a valuable tool for non-destructive depth profiling. The experimentally determined values of the inelastic electron mean free path in Co2MnSi increase from about 19.5 to 67 Å on increasing the kinetic energy from about 1.9 to 6.8 keV. The influence of the thermal treatment of Co2MnSi thin films on the electronic properties was also explored. The structure of the thin films is significantly improved by heat treatment as revealed by x-ray diffraction. It was found that the electronic structure of annealed samples as measured by photoelectron spectroscopy is similar to that of a well-ordered bulk reference sample. The samples without heat treatment show strong deviations from the electronic structure of bulk material. The differences between the disordered and the ordered films are also observed in core level spectra. Chemical shifts of about 100 meV are observed at the Mn 2p states. The stronger localization of the Mn d states in the ordered samples is obvious from the multiplet satellite of the Mn 2p3/2 state.