Defect-induced States Research Articles

Rearrangement of π-Electron Network and Switching of Edge-Localized π State in Reduced Graphene Oxide

Introduced defects can modulate the intrinsic electronic structure of graphene, causing a drastic switch in its electronic and magnetic properties, in which defect-induced localized π states near the Fermi level play an important role. Accordingly, considerable effort has been directed toward detailed characterization of the defect-induced state; however, identification of the chemical nature of the defect-induced state remains a challenge. Here, we demonstrate a method for reliable identification of the localized π states of oxidized vacancy edges in reduced graphene oxide. Depending on the dynamic changes in the oxygen-binding modes, i.e., between carbonyl and ether forms in the vacancy edges, the π-electron network near the edges can rearrange, leading to drastic on-off switching of the localized π state. This switching can be manipulated via scanning-probe-induced local mechanical force. This study provides fundamental guidance toward understanding how oxidized defect structures contribute to the unique electronic state of graphene oxide and its potential future applications in electronic devices.

Oxygen vacancy formation and the induced defect states in HfO2 and Hf-silicates – A first principles hybrid functional study

O vacancy formation and the associated electronic defect states in Hf-based oxides have been investigated over a wide range of compositions using first-principles hybrid density functional calculations. Based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device.

Hopping transport through defect-induced localized states in molybdenum disulphide

Molybdenum disulphide is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. Here we report a study of transport in few-layer molybdenum disulphide, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulphur vacancies exist in molybdenum disulphide, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbour hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of molybdenum disulphide.

Point defect-induced transport bandgap widening in the downscaled armchair graphene nanoribbon device

We report on ab initio study of the electronic states and transport characteristics for armchair graphene nanoribbon devices with point defects. Geometric optimization of the point defect channel revealed the self-organizing property of the graphene. Density of state (DOS) calculation shows the point defect-induced gap states. However, the quantum transport calculations revealed that these defect-induced gap states are not contributing to the carrier transport across the channel. This is clarified further from the wavefunction analysis, which shows the spatial localization of the wavefunction around the defect regions. Most importantly, the widening of the transport bandgap with the increase in number of point defects was found even though the DOS bandgap remains unchanged. The impact of these point defects on the device characteristics varies depending on their type and location in the channel. The orientation of the defects plays a vital role in the carrier scattering, which is a crucial factor to be considered in downscaled devices.

Integrating MBE materials with graphene to induce novel spin-based phenomena

Magnetism in graphene is an emerging field that has received much theoretical attention. In particular, there have been exciting predictions for induced magnetism through proximity to a ferromagnetic insulator as well as through localized dopants and defects. Here, the authors discuss their experimental work using molecular beam epitaxy to modify the surface of graphene and induce novel spin-dependent phenomena. First, they investigate the epitaxial growth of the ferromagnetic insulator EuO on graphene and discuss possible scenarios for realizing exchange splitting and exchange fields by ferromagnetic insulators. Second, they investigate the properties of magnetic moments in graphene originating from localized pz-orbital defects (i.e., adsorbed hydrogen atoms). The behavior of these magnetic moments is studied using nonlocal spin transport to directly probe the spin-degree of freedom of the defect-induced states. They also report the presence of enhanced electron g-factors caused by the exchange fields present in the system. Importantly, the exchange field is found to be highly gate dependent, with decreasing g-factors with increasing carrier densities.

Surface solitons of four-wave mixing in an electromagnetically induced lattice

By creating lattice states with two-dimensional spatial periodic atomic coherence, we report an experimental demonstration of generating two-dimensional surface solitons of a four-wave mixing signal in an electromagnetically induced lattice composed of two electromagnetically induced gratings with different orientations in an atomic medium, each of which can support a one-dimensional surface soliton. The surface solitons can be well controlled by different experimental parameters, such as probe frequency, pump power, and beam incident angles, and can be affected by coherent induced defect states.

Electrically tunable molecular doping of graphene

Electrical tunability of molecular doping of graphene has been investigated using back-gated field effect transistors. Variation of the gate voltage from positive to negative values resulted in reduced p-type doping by NO2, which decreased below detection limit at −45 V. A reverse trend was observed for NH3, where its n-type doping increased with more negative gate voltage, becoming undetectable at 5 V. Our results indicate that adsorption induced molecular doping of graphene could not be detected when the Fermi level coincides with the adsorption induced defect states, which yields NO2 acceptor energy level of ∼320 meV below the Dirac point.

Mn-Doped Multinary CIZS and AIZS Nanocrystals.

Multinary nanocrystals (CuInS2, CIS, and AgInS2, AIS) are widely known for their strong defect state emission. On alloying with Zn (CIZS and AIZS), stable and intense emission tunable in visible and NIR windows has already been achieved. In these nanocrystals, the photogenerated hole efficiently moves to the defect-induced state and recombines with the electron in the conduction band. As a result, the defect state emission is predominantly observed without any band edge excitonic emission. Herein, we report the doping of the transition-metal ion Mn in these nanocrystals, which in certain compositions of the host nanocrystals quenches this strong defect state emission and predominantly shows the spin-flip Mn emission. Though several Mn-doped semiconductor nanocrystals are reported in the literature, these nanocrystals are of its first kind that can be excited in the visible window, do not contain the toxic element Cd, and provide efficient emission. Hence, when Mn emission is required, these multinary nanocrystals can be the ideal versatile materials for widespread technological applications.

Ultraviolet emission efficiency enhancement of a-plane AlGaN/GaN multiple-quantum-wells with increasing quantum well thickness

The defect-induced carrier localization in nonpolar a-plane (Al,Ga)N/GaN multiple quantum wells (MQWs) structures with different well thickness have been investigated. A strong variation of temperature-dependent photoluminescence peak energy was observed and attributed to the existence of the localized states. The degree of carrier localization in these defect-induced states was more prominent in the case of MQWs with the wider well width. In addition, the ultraviolet light emission efficiency revealed a 3-fold enhancement with increasing the well width from 1.6 nm to 7.3 nm, due to the strong carrier localization generated from the quantum-wire-like features formed by the intersection between basal stacking faults and quantum wells.

ELECTRONIC, MAGNETIC, AND MECHANICAL PROPERTIES OF LINE-DEFECT EMBEDDED GRAPHENE NANORIBBONS: A FIRST-PRINCIPLES STUDY

We use first-principles method to investigate the electronic, magnetic, and mechanical properties of graphene nanoribbons (GNRs) with extended line defect. Particular attention has been placed on zigzag GNR with 5-8-5 line defect and armchair GNR with 4-8 line defect. The results show that the band gaps of GNRs can be effectively tuned by line defect, which depend on both their widths and the position of defect. The line-defect embedded GNRs are either metals or semiconductor with markedly reduced band gap. The band-gap reduction is attributed to the defect-induced impurity states. In particular, the metallic line-defect embedded zigzag GNRs are ferromagnetic at ground state, while those semiconducting ones are antiferromagnetic. Upon the line-defect embedded armchair GNRs, the band gaps vary periodically with the increasing widths. Our results imply the potential applications of line-defect embedded GNRs at nanoscale electronics.

Vacancies in GaN bulk and nanowires: effect of self-interaction corrections

We investigate gallium and nitrogen vacancies in gallium nitride (GaN) bulk and nanowires using self-interaction corrected pseudopotentials (SIC). In particular, we examine the band structures to compare and contrast differences between the SIC results and standard density functional theory (DFT) results using a generalized gradient approximation (GGA) (Perdew et al 1996 Phys. Rev. Lett. 77 3865) functional. For pure nanowires, we observed similar trends in the bandgap behaviour, with the gap decreasing for increasing nanowire diameters (with larger bandgaps using SIC pseudopotentials). For gallium vacancies in bulk GaN and GaN nanowires, SIC results are similar to DFT-GGA results, albeit with larger bandgaps. Nitrogen vacancies in bulk GaN show similar defect-induced states near the conduction band, whilst a lower lying defect state is observed below the valence band for the DFT-GGA calculations and above the valence band for the SIC results. For nitrogen vacancies in GaN nanowires, similar defect states are observed near the conduction band, however, while the SIC calculations also show a defect state/s above the valence band, we were unable to locate this state for the DFT-GGA calculations (possibly because it is hybridized with edge states and buried below the valence band).

Role of defect electronic states in the ferromagnetism in graphite

The mechanism of ferromagnetism in carbon-based materials, which contain only $s$ and $p$ electrons in contrast to traditional ferromagnets based on $3d$ or $4f$ electrons, is important but unclear. Here, by means of near-edge x-ray-absorption fine-structure (NEXAFS) and bulk magnetization measurements, we demonstrate that the origin of ferromagnetism in ${}^{12}$C${}^{+}$ ion implanted highly oriented pyrolytic graphite (HOPG) is closely correlated with the defect electronic states near the Fermi level. The angle-dependent NEXAFS spectra imply that these defect-induced electronic states are extended on the graphite basal plane. We attribute the origin of electronic states to the vacancy defects created under ${}^{12}$C${}^{+}$ ion implantation. The intensity of the observed ferromagnetism in HOPG is sensitive to the defect density, and the narrow implantation dosage window that produces ferromagnetism is optimized.

First principles study of the oxygen vacancy formation and the induced defect states in hafnium silicates

Using first-principles density functional theory calculations, we have investigated the O vacancy formation and the relevant induced defect states in hafnium silicates over a wide range of compositions. The PBE0 hybrid density functional was employed for the analysis of the electronic properties and the charge transition levels of the O vacancy in crystalline HfSiO4 and in amorphous Hf-silicates, respectively. Based on the generated structure models, eight typical kinds of O coordination structures were identified in amorphous Hf-silicates. Our calculated results show that the positions of the induced defect energy levels in the band gap and the formation energies of O vacancy are largely determined by the local structures of the vacancy sites, which appear to be nearly independent of the composition of amorphous Hf-silicates. Our calculations also show that O vacancy can possess the negative-U behavior in crystalline HfSiO4 but not in amorphous Hf-silicates, where most of the O vacancies can simply exhibit the negative-U behavior as in the positive charge states. Given the measured band offset of 3.40 eV between Si and amorphous Hf-silicates, a considerable number of O vacancies were found to prefer to stay in the charge neutral state as the Fermi level lies within the band gap region of Si. Furthermore, due to its relatively higher formation energy, the concentration of O vacancy in Hf-silicates can be much lower than that in m-HfO2 when the Fermi level lies below the midgap region of Si. Accordingly, a significantly reduced flat band voltage shift and less transient threshold voltage instability can be found in Hf-silicates as compared with m-HfO2, which are in good agreement with the recent experimental findings.

Defect-induced solid state amorphization of molecular crystals

We investigate the process of mechanically induced amorphization in small molecule organic crystals under extensive deformation. In this work, we develop a model that describes the amorphization of molecular crystals, in which the plastic response is calculated with a phase field dislocation dynamics theory in four materials: acetaminophen, sucrose, γ-indomethacin, and aspirin. The model is able to predict the fraction of amorphous material generated in single crystals for a given applied stress. Our results show that γ-indomethacin and sucrose demonstrate large volume fractions of amorphous material after sufficient plastic deformation, while smaller amorphous volume fractions are predicted in acetaminophen and aspirin, in agreement with experimental observation.

Optical investigation on cadmium-doped zinc oxide nanoparticles synthesized by using a sonochemical method

Cadmium-doped zinc oxide CdxZn1−xO (x = 0, 0.05, 0.15 and 0.40) nanoparticles were synthesized by using a sonochemical method. The X-ray diffraction (XRD) patterns show that CdxZn1−xO nanoparticles keep a wurtzite structure up to x = 0.40. However, the lattice parameters a, c, and internal parameter u gradually increase with increasing x. The Raman scattering investigation indicates that the Raman mode shows a significant softening in Cd-doped ZnO nanoparticles with increasing x, which suggests an expansion of the Zn–O bond. We also observe a significant red-shift of the band gap for CdxZn1−xO nanoparticles as x increases, i.e., the gap energy Eg ∼ 3.21 eV for x = 0 shifts to 2.92 eV for x = 0.40. In addition, the Urbach band tail gradually increases with x-value, which indicates an enhancement of the defect states in samples with Cd incorporation. In CdxZn1−xO nanoparticles, we further find that the near band-edge (NBE) emission band shows a red-shift with increase in x-value. A broad photoluminescence (PL) band is also observed at around 590 nm, which enhances from x = 0 to 0.15. With further increases in the x-value to 0.40, a weak PL band appears around 450 nm while the broad band around 590 nm is suppressed. The variation of PL intensity is ascribed to the change of the Cd-doping induced defect states and the nonradiative energy transfers.

X-ray nanotomography of SiO2-coated Pt90Ir10tips with sub-micron conducting apex

Hard x-ray nanotomography provides an important three-dimensional view of insulator-coated “smart tips” that can be utilized for modern emerging scanning probe techniques. Tips, entirely coated by an insulating SiO2 film except at the very tip apex, are fabricated by means of electron beam physical vapor deposition, focused ion beam milling and ion beam-stimulated oxide growth. Although x-ray tomography studies confirm the structural integrity of the oxide film, transport measurements suggest the presence of defect-induced states in the SiO2 film. The development of insulator-coated tips can facilitate nanoscale analysis with electronic, chemical, and magnetic contrast by synchrotron-based scanning probe microscopy.

Atomic and Electronic Properties of Realizable Size Single-Crystal GaN Nanotubes by First Principles

We studied the diameter and wall thickness dependent atomic and electronic properties of practical size single-crystal GaN nanotubes using first principle calculations. Single-crystal GaN nanotubes are similar to the hexagonal GaN nanowires, grown in the [0001] direction with [10-10] facets, except there is an axial hexagonal void in them. We first demonstrated that the atomic and electronic properties of these tubes are mainly determined by the thickness of their wurtzite walls; and their diameters have negligible effects. Then, considering the individual walls of GaN nanotubes in two-dimensional slab calculations we examine the bond distances, formation energy, band gap, effective electron mass and the evolution of electronic density of the states as a function of thickness for unsaturated and hydrogen-saturated slabs of GaN. Calculations revealed that the unsaturated dangling bonds at the surfaces induce defect states in the band gap region of unsaturated tubes. Therefore, regardless of diameter and wall thickness, their band gaps are always smaller than that of the bulk GaN. However, the band gaps of the hydrogen-saturated tubes are found to be amplified with respect to bulk GaN. The amplification in the band gaps as a function of wall thickness in the range of 5.6-16.9 A and 16.9-28.1 A scales with a factor of 1/d(0.9281) and 1/d(1.769), respectively. Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness as small as 28.1 A would be stable and they would have a noticeably larger band gap with respect to the band gap of bulk GaN.

Interaction of oxygen vacancies and lanthanum in Hf-based high-k dielectrics: an ab initio investigation

The interaction between oxygen vacancies and La atoms in the La doped HfO2 dielectric were studied using first principles total energy calculations. La dopants in the vicinity of a neutral oxygen vacancy (V O) show lower formation energy compared to the La defects far from V O centres. La doping in HfO2 leads to the shift of the defect states of oxygen vacancies towards the conduction band edge. A statistical average of this shift over several possible configurations of La atoms and V O shows that the incorporation of La effectively passivates the V O induced defect states leading to the reduction of the gate leakage current and improvement of the device reliability.

Erratum: Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations [Phys. Rev. Lett.106, 206803 (2011)

Erratum: Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations [Phys. Rev. Lett.<b>106</b>, 206803 (2011)

Identification and elimination of inductively coupled plasma-induced defects in AlxGa1-xN/GaN heterostructures

By using temperature-dependent Hall, variable-frequency capacitance—voltage and cathodoluminescence (CL) measurements, the identification of inductively coupled plasma (ICP)-induced defect states around the AlxGa1-xN/GaN heterointerface and their elimination by subsequent annealing in AlxGa1-xN/GaN heterostructures are systematically investigated. The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed. The interface state density after the ICP-etching process is as high as 2.75×1012 cm-2·eV-1. The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N2 ambient. The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.