Decrease In Work Function Research Articles

Effect of Surface Chemisorption between Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) and Ag Nanoparticles on the Conductivity of the Nanocomposite Film

Abstract The effects of Ag nanoparticle (NP) incorporation on the bonding state and electronic properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films were investigated. Surface stabilization of the Ag NPs by trisodium citrate effectively prevented aggregation of the Ag NPs. The introduction of Ag NPs induced PEDOT moieties as chemisorption centers. The resulting PEDOT-rich film surface with Ag NPs was responsible for the enhanced film conductivity and decreased work function of the Ag NP–PEDOT:PSS film.

In situformation and electron-spectroscopic study of bis(arene) V and Cr compounds on a graphite surface

Thin layers of bis(arene) transition metal (TM) compounds have been formed in situ in ultra-high vacuum by depositing Cr or V metal on top of a film of benzene (Bz) or toluene (Tol) ice at 100 K on a graphite substrate. The species thus formed are Cr(C6H6)2, V(C6H6)2, or Cr(C6H5-CH3)2 (termed CrBz2, VBz2, or Cr(Tol)2), and the valence structures have been examined using electron energy loss and ultraviolet photoemission spectroscopies (ELS and UPS). The reaction is “clean” in that there is no indication of side reactions or unwanted by-products, although a small coverage of unreacted TM atoms appears unavoidable. A simple ring substituent (CH3) remains intact during the reaction. Thus, it should be possible to synthesize and study species that are not readily available (or cannot easily be made) in bulk form or that have too low vapor pressure to allow easy in situ deposition. Data have been obtained for VBz2 and Cr(Tol)2, the valence structures of which have not previously been studied in detail using UPS and ELS. Although deeper-lying arene orbitals are essentially unperturbed by formation of the bis(arene)TM sandwich, subtle differences in the TM-related orbitals have been observed and discussed in comparison to CrBz2 with the aid of density functional theory. In the case of VBz2, clear evidence is seen for a molecular reorientation during annealing, based on the ELS polarization dependence. Electron transfer between an adsorbed bis(arene)TM and graphite depends on the occupancy of the highest occupied molecular orbital (HOMO) and on its energy relative to the Fermi level (EF). The small amount of unreacted TM resulting from in situ synthesis causes a large decrease in work function, which shifts the adsorbate HOMO to below EF and thus impedes electron transfer to highly oriented pyrolytic graphite. This is an obstacle to forming doping layers by this method.

Synthesis and field emission properties of highly ordered Ti-doped ZnO nanoarray structure

In the present paper, we synthesized highly ordered titanium (Ti)-doped ZnO nanorod arrays utilizing a facile hydrothermal method and a polystyrene nanosphere monolayer mask. The crystalline phase and morphology of the as-prepared samples were investigated systematically. The field emission (FE) properties measurement demonstrated that the Ti-doped ZnO nanorod arrays exhibited a significantly improved field enhancement factor β as well as a lowered turn-on field and a lowered threshold field. These improved FE performances were attributed to the decreased work function which resulted from the widened energy gap combined with the up-shift of Fermi level in n-type Ti-doped ZnO nanorod arrays.

Zn12O12 nano-cage as a promising adsorbent for CS2 capture

Density functional theory (DFT) calculations were performed to investigate the adsorption of CS2 molecule on the Zn12O12 nano-cage. The adsorption energy in the most stable configuration is −1.39eV and the HOMO–LUMO energy gap (Eg) of the nano-cage is decreased from 4.19 to 3.67eV upon the CS2 adsorption. The Eg of the nano-cage is decreased by increasing the number of the CS2 molecules. Also, it was found that the field electron emission from the Zn12O12 surface may be improved upon the adsorption of CS2 molecules due to decrease of work function. The strong adsorption of the CS2 on the Zn12O12 nano-cage indicates the potential application of the nano-cage for CS2 capture and storage.

Effect of Ar+ ion etching treatment on the surface work function of Hg3In2Te6 wafer

Synchrotron radiation photoemission spectroscopy (SRPES) was employed to investigate the changes of surface work function of Hg3In2Te6 wafer under different Ar+ ion etching parameters. Hg concentration on the Hg3In2Te6 surface decreased with increasing the etching time. Meanwhile, the calculated work function and electron affinity of Hg3In2Te6 wafer also decreased with increasing the etching time. It was found that the decrease of work function of the Hg3In2Te6 wafer was mainly caused by the decrease of electron affinity which was closely related to the increase of electron concentration due to the escape of Hg2+ on the surface of Hg3In2Te6 wafer.

Molecular and electronic structure of electroactive self-assembled monolayers

Self-assembled monolayers (SAMs) containing electroactive functional groups are excellent model systems for the formation of electronic devices by self-assembly. In particular ferrocene-terminated alkanethiol SAMs have been extensively studied in the past. However, there are still open questions related with their electronic structure including the influence of the ferrocene group in the SAM-induced work function changes of the underlying metal. We have thus carried out a thorough experimental and theoretical investigation in order to determine the molecular and electronic structure of ferrocene-terminated alkanethiol SAMs on Au surfaces. In agreement with previous studies we found that the Fc-containing alkanethiol molecules adsorb forming a thiolate bond with the Au surface with a molecular geometry 30° tilted with respect to the surface normal. Measured surface coverages indicate the formation of a compact monolayer. We found for the first time that the ferrocene group has little influence on the observed work function decrease which is largely determined by the alkanethiol. Furthermore, the ferrocene moiety lies 14 Å above the metal surface covalently bonded to the alkanethiol SAM and its HOMO is located at -1.6 eV below the Fermi level. Our results provide new valuable insight into the molecular and electronic structure of electroactive SAMs which are of fundamental importance in the field of molecular electronics.

(Invited) III-V/Oxide Interfaces Investigated with Synchrotron Radiation Photoemission Spectroscopy

We used synchrotron radiation photoemission spectroscopy (SR-PES) to investigate the surface of GaAs and other III-V semiconductors after tri-methyl-aluminum (TMA) pulses performed at 250{degree sign}C. We observed the removal of native oxide and the growth of Al-oxide upon measuring the As3d, Ga3d, Al2p and VB spectra. After seven TMA pulses we performed one water pulse. As3d and Ga3d peaks showed almost no lineshape change, but a decrease of intensity due to the water adsorption. The valence band shows a change in the secondary electron cut-off and the decrease of work function. We conclude that water mostly adsorbs molecularly and induces the work function decrease. The molecular absorption of water indicates that the seven TMA pulses produce a passivated surface only partially terminated with OH groups.

Comparison of CsBr and KBr covered Cu photocathodes: Effects of laser irradiation and work function changes

Thin films of CsBr and KBr were deposited on Cu(100) to investigate photoemission properties of these potential photocathode materials. After prolonged laser ultraviolet irradiation photoemission, quantum efficiency increases by factors of 26 and 77 for KBr/Cu(100) and CsBr/Cu(100) photocathodes, respectively. Immediately following thin film deposition, a decrease in work function is observed, compared to bare Cu, in both cases. Quantum efficiency enhancements are attributed to the decrease in photocathode work function, and photo-induced processes that introduce defect states into the alkali halide bandgap. Our results suggest that KBr, a relatively stable alkali-halide, also has potential for photocathode applications.

Simple chemical synthesis of porous carbon spheres and its improved field emission by water vapor adsorption

A very simple solvothermal approach for synthesizing porous carbon microspheres is reported here. The effect of water adsorption on the field emission properties of such samples was investigated experimentally as well as from first principle calculation. The as-synthesized microspheres were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (TEM), atomic force microscopy and Fourier transformed infrared (FTIR) spectroscopy. XRD patterns as well as TEM lattice images confirmed that the spheres were graphitic in nature. From the microscopic analyses it was seen that the as-synthesized porous carbon spheres were actually composed of few layer thick carbon nanosheets. FTIR confirmed the adsorption of water in the sample. It has been found that samples with adsorbed water can act as a better field emitter compared to the pure ones. A first principle study showed that although no development of H2O-induced impurity states close to the Fermi level was observed due to water adsorption, there was a pronounced upward shift in the Fermi level, i.e. a decrease of work function, which is believed to be the main reason for the observed improved field emission performance.

Experimental and computational insights in the growth of gallium-doped zinc oxide nanostructures with superior field emission properties

Well-aligned, single-crystalline Ga-doped zinc oxide nanopagoda arrays were fabricated on silicon substrates via a metal–organic chemical vapor deposition method. Gallium atoms played a crucial role in transforming the lateral facets of nanorods from {100} to two sets of facets: ({111}, {11}) and ({201}, {10}), which eventually led to a pagoda shape. Based on computational simulation results, gallium lowers the surface energies of the ({111}, {11}) and ({201}, {10}) planes but increases that of the {100} plane. We proposed a new growth model, which involves the change of surface energy calculated by computational simulation due to Ga doping, to interpret why the smooth {100} planes of nanorods transform to corrugated ({111}, {11}) and ({201}, {10}) planes of nanopagodas. The nanopagodas not only possess excellent crystal quality but also exhibit remarkable field emission properties. Field emitters made of Ga-doped ZnO nanopagodas had a low turn-on field due to the decrease of work function and the increase of conductivity caused by Ga; simultaneously, the interesting pagoda shape enhanced significantly the field emission β values by 18 times.

Growth of flexible N-doped SiC quasialigned nanoarrays and their field emission properties

In the present work, we report the growth of flexible SiC quasialigned nanoarrays with N dopants on carbon fabric substrate via the catalyst-assisted pyrolysis of a polymeric precursor. The resultant products are systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS). The as-synthesized SiC nanowires are single-crystalline and grow along the [111] direction with a uniform spatial distribution of N dopants. The effect of the distance between the SiC array and the anode on the Field emission (FE) properties was investigated. FE measurements show that these N-doped SiC nanoarrays could be an excellent candidate for field emitters with very low turn-on fields of 1.90–2.65 V μm−1 and threshold fields of 2.53–3.51 V μm−1, respectively, which can be mainly attributed to the decrease of work function induced by the N dopants.

First-principles study of field emission properties for ZnO nanotuber capped and codoped with N-M(Cd, Mg)

The first-principles density-functional theoretical calculations are performed to investigate the effects of N doped and N, M (Cd, Mg) codoped on the geometrical structures and field emission properties of capped (9, 0) zinc oxide nanotubes (ZnONT). The results show that the N could improve the stability of the structure of capped side. With the increase of the applied electric field, the density of states (DOS) shifts towards the low energy position, the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and the effective work function decrease drastically, and the electrons congregate to the capped side. The analyses of DOS/local DOS, HOMO/LUMO, and Mulliken population indicate that the field emission properties of N, Cd-Codoped ZnONT are improved, but those of N, Mg-codoped ZnONT are worsened.

Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates

A chemical approach was applied to decrease the work function of few-layer graphene. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 to 1700–2500 Ω/sq, and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 to 96.1–94% due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for graphene doped with Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, respectively. This suggested that spontaneous chemical combination occurred between the carbon atoms and alkali metal, thereby decreasing the work function.

Effect of Silica Nanoparticle Content on the Structure and Electrostatic Bonding of PEDOT:PSS

Conducting poly(3,4-ethylenedioxythiophene):poly(styrene-4-sulfonate) (PEDOT:PSS) containing hydroxyl surface rich SiO2 nanoparticles were investigated. Rearrangement and bonding of PEDOT:PSS were affected by the content of SiO2 nanoparticles, which resulted in the modification of an electronic band structure and electrical characteristics. A slight decrease in work function and a shift of the highly occupied molecular orbital edge to the Fermi level were induced by SiO2 nanoparticle addition. The current path was also tuned by SiO2 nanoparticles and resistance of the composite film was diminished. SiO2 nanoparticles affect electrostatic bonding between PEDOT:PSS and doping of PEDOT as well as increase the packing density of PEDOT:PSS.

Covalent functionalization of AlN nanotubes with acetylene

Covalent functionalization of a zigzag AlN nanotube (AlNNT) with acetylene molecule has been investigated using density functional theory in terms of energetic, geometry and electronic properties. The obtained results show that the acetylene preferably tends to [2+2] addition on Al–N bonds of the tube surface releasing energies in the range of 1.02 to 1.10eV along with a charge transfer from the tube to the acetylene. We have found that HOMO–LUMO gap of AlNNT has not changed significantly upon the functionalization and, its global hardness and electrophilicity are slightly decreased. Also, it has been revealed that the field electron emission from the AlNNT surface may be improved upon the adsorption of acetylene due to decrease of work function ascribed to the charge transfer from the tube to the acetylene.

Lateral interaction in Be and Mg layers on the Mo(112) surface

The formation of structures of Be and Mg on the furrowed Mo(112) surface has been studied by DFT. Both Be and Mg adsorb in furrows. The adsorption of small Be adatoms results in the smoothing of the surface and therefore increases the work function. The dominant lateral interaction for Be on Mo(112) is the attraction between nearest-neighbor Be adatoms, which, at low coverages, leads to the formation of islands of the p(1 × 1) structure. In contrast, larger Mg adatoms are polarized, so that the work function decreases. Friedel oscillations of screening electrons, enhanced due to existence of flat segments of Fermi surface, provide a favorable decay asymptotic of the indirect interaction along the furrows, which explains the formation of the p(4 × 1) chain structure in Mg layers on Mo(112).

Thickness-dependent field emission from ZnTe films prepared by magnetron sputtering

The thickness effect on the field emission properties of ZnTe films deposited on silicon substrates by magnetron sputtering is investigated. All the films exhibit amorphous structure with nanocrystalline embedded. As the thickness increases, the surface becomes much smoother, while the Zn/Te ratio, the turn-on field and the work function decrease. Their field-emission characteristics show a low turn-on field of 7.5Vμm−1 and a high current density of 67μAcm−2 at an electric field of 8.9Vμm−1. The thickness-dependent electron field emission phenomenon is explained by a space-charge-induced bandbending interlayer model.

Cathode Work Function Dependence of Electron Transport Efficiency through Buffer Layer in Organic Solar Cells

An electron transport mechanism through a cathode buffer layer of organic solar cells is experimentally investigated. Inverted organic solar cells with the structure of indium–tin oxide (ITO)/thin cathode metal/bathocuproine (BCP)/fullerene (C60)/copper phthalocyanine (CuPc)/pentacene/Ag (anode) are examined. A new model, in that electrons are transported across the BCP buffer layer not through defect states but over the lowest unoccupied molecular orbital (LUMO), is proposed. That is, the defect state density in the BCP layer is not important for electron transport, though the hopping transport model via the defect states is widely accepted. The transport efficiency is sensitive to the cathode work function (WFC). As WFC decreases, the energy difference between the BCP LUMO and the cathode Fermi level decreases. This introduces a low electronic potential barrier height from the cathode to the acceptor. The low WFC is thus important to achieve a low series resistance. Furthermore, the dependence of WFC on barrier height is also confirmed for tris(8-hydroxyquinolinato)aluminum buffer.

Solution processed sodium chloride interlayers for efficient electron extraction from polymer solar cells

We investigate a solution-processable, non-toxic sodium chloride (NaCl) interlayer for aluminum cathodes in organic solar cells. The electron extraction at the cathode interface is strongly improved upon the insertion of NaCl leading to power conversion efficiencies of up to 2.9% as compared to 1.8% efficient devices without interlayer. Scanning electron and kelvin probe force microscopy studies reveal that the formation of NaCl crystals causes a decrease of the aluminum work function by more than 300 mV. By impedance spectroscopy, we found evidence that the NaCl crystals improve the energetic alignment at the polymer/metal interface.

Electrical Properties and Reliability of ZnO-Based Nanorod Current Emitters

The fabrication, optical, and field emission properties of ZnO-based nanorod emitters were studied. Ga-doped ZnO nanorods combined with the formation of tip structure on top of a ZnO nanorod by oxygen plasma treatment are employed to improve the field emission properties of the nanorod emitters. By either of these two methods, the nanorod emitters exhibit significantly reduced turn-on field and enhanced field-emission factor. The morphology, crystal structure, and composition of all the nanorods used for making emitters are characterized by a scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectrometer. The nanorods exhibit the highly preferred c -axis orientation single crystal structure. The photoluminescence spectra indicate the nanorods have better crystalline structure after doping and oxygen plasma treatment. Combining gallium doping process (Ga/Zn molar ratio of 1% in solution) and oxygen plasma treatment (etching time of 60 s), the tip-structured GZO nanorod emitters with tip angle of 100° have a turn-on field of 1.99 V/μm under a current density of 1 μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , field enhancement factor of 2465, and stable operation over 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s. Such improved field emission properties are attributed to decreased work function and sharp nanotips morphology. In addition, the GZO nanorod emitters with tip structure are successively and stably operated between 25°C and 100°C over 3000 s based on the high-temperature field emission measurement results. They have high potential for practical applications in flat panel display and light emitting device in the future.