Surface transfer p-type doping of epitaxial graphene

Epitaxial graphene thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based high-resolution photoemission spectroscopy reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This novel surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to nondestructively dope epitaxial graphene for future nanoelectronics applications.     

Surface transfer doping of semiconductors

Surface transfer doping relies on charge separation at interfaces, and represents a valuable tool for the controlled and nondestructive doping of nanostructured materials or organic semiconductors at the nanometer-scale. It cannot be easily achieved by the conventional implantation process with energetic ions. Surface transfer doping can effectively dope semiconductors and nanostructures at relatively low cost, thereby facilitating the development of organic and nanoelectronics. The aim of this review is to highlight recent advances of surface transfer doping of semiconductors. Special focus is given to the effective doping of diamond, epitaxial graphene thermally grown on SiC, and organic semiconductors. The doping mechanism of various semiconductors and their possible applications in nanoelectronic devices will be discussed, including the interfacial charge transfer and the energy level alignment mechanisms.     

Surface transfer doping of diamond: A review

Ultra-wide bandgap materials show great promise as a solution to some of the limitations of current state of the art semiconductor technology. Among these, diamond has exhibited great potential for use in high-power, high-temperature electronics, as well as sensing and quantum applications. Yet, significant challenges associated with impurity doping of the constrained diamond lattice remain a primary impediment towards the development of diamond-based electronic devices. An alternative approach, used with continued success to unlock the use of diamond for semiconductor applications, has been that of ‘surface transfer doping’ - a process by which intrinsically insulating diamond surfaces can be made semiconducting without the need for traditional impurity doping. Here, we present a review of progress in surface transfer doping of diamond, both a history and current outlook of this highly exploitable attribute.     

Synthesis and characterization of a metal-to-polyoxometalate charge transfer molecular chromophore

[P(4)W(35)O(124){Re(CO)(3)}(2)](16-) (1), a Wells-Dawson [α(2)-P(2)W(17)O(61)](10-) polyoxometalate (POM)-supported [Re(CO)(3)](+) complex containing covalent W(VI)-O-Re(I) bonds has been synthesized and characterized by several methods, including X-ray crystallography. This complex shows a high visible absorptivity (ε(470 nm) = 4000 M(-1) cm(-1) in water) due to the formation of a Re(I)-to-POM charge transfer (MPCT) band. The complex was investigated by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the Re(I) center to the POM ligand.     

Observation of intermolecular charge transfer in a quasi-one-dimensional molecular alloy system

X-band single-crystal electron paramagnetic resonance (EPR) studies of the molecular alloy [NO 2BzPy][Au 0.57Ni 0.43(mnt) 2] are presented in this paper. At room temperature, EPR spectra show both intense resonance signals (main signals) and weak satellite quartet lines. The characteristics of both intense and weak EPR signals depend on the magnetic field orientation. The main signals arise from two magnetically nonequivalent [Ni(mnt) 2] (-) anions, and their corresponding principal values of the g tensor are ( g x ') 1 = 2.04653, ( g y ') 1 = 2.00096, and ( g z ') 1 = 2.15319 and ( g x ') 2 = 2.04520, ( g y ') 2 = 1.99734, and ( g z ') 2 = 2.15361, respectively. The weak satellite lines, whose patterns strongly depend on the magnetic field direction, can be attributed to the hyperfine coupling of the electron spin with the (197)Au nucleus of the [Au(mnt) 2] (-) species. Density functional theory calculations for the spin and charge distributions of the dimer {[Ni(mnt) 2][Au(mnt) 2]} (2-) indicate that the hyperfine interaction of the electron spin with the (197)Au nuclear spins is caused, in part, by the charge transfer between the [Ni(mnt) 2] (-) and the [Au(mnt) 2] (-) species.     

Photoinduced charge transfer in molecular materials studied by optical absorption using photoacoustic spectroscopy

Photoacoustic spectra of molecular materials based on the assembling of the [Fe(CN)₆] molecular block were recorded and evaluated. Those compositions where the valence of the involved metals allows a charge transfer (an inner photoinduced redox reaction) through the CN ligand shown an intense photoacoustic signal around 600 nm; when this transition is unable only the signal corresponding to metal-to-ligand and d-d transitions within the metal were observed. This suggests that this technique provides a fast and reliable method to explore the existence of tunable photoinduced charge transfer in molecular materials.     

Positron annihilation studies of some charge transfer molecular complexes

Positron annihilation lifetimes were measured for some solid charge transfer (CT) molecular complexes of quinoline compounds (2,6-dimethylquinoline, 6-methoxyquinoline, quinoline, 6-methylquinoline, 3-bromoquinoline and 2-chloro-4-methylquinoline) as electron donor and picric acid as an electron acceptor. The infrared spectra (IR) of the solid complexes clearly indicated the formation of the hydrogen-bonding CT-complexes.

The annihilation spectra were analyzed into two lifetime components using PATFIT program. The values of the average and bulk lifetimes divide the complexes into two groups according to the non-bonding ionization potential of the donor (electron donating power) and the molecular weight of the complexes. Also, it is found that the ionization potential of the donors and molecular weight of the complexes have a conspicuous effect on the average and bulk lifetime values. The bulk lifetime values of the complexes are consistent with the formation of stable hydrogen-bonding CT-complexes as inferred from the IR-spectral data.     

Superstructures of donor packing arrangements in a series of molecular charge transfer salts

Three conducting BEDT-TTF charge-transfer salts with tris(oxalato)metallate anions have unit cells containing both[small alpha] and [small beta][double prime] donor packing motifs.     

Theory of vibrationally coupled pairwise charge transfer in a linear conjugated molecular chain

A model describing the electron charge transfer along a linear conjugated molecular chain based on a double harmonic oscillator picture is presented. Qualitatively, the model predicts three temperature regions of distinct conductivity characteristics: semiconductor-like (low temperature, TR-A), metal-like (high temperature, TR-C) and a narrow transition temperature region (TR-B) between the above two. In ideal cases, where the π-bond transfer frequency ω, coincides with the antisymmetric normal model of vibration frequency, υ, a cooperative pairwise charge transfer (“covalon”) is expected to take place leading to an anomalously high conductivity.     

Interfacial electron transfer in FeII(CN)6 4- -sensitized TiO2 nanoparticles: a study of direct charge injection by electroabsorption spectroscopy

Electroabsorption (Stark) spectroscopy has been used to study the dye sensitized interfacial electron transfer in an Fe(II)(CN)(6)(4)(-) donor complex bound to a TiO(2) nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 A. This value is similar to the estimated distance between the Fe(II) center of the complex and the Ti(IV) surface site coordinated to the nitrogen end of a bridging CN ligand in (CN)(5)Fe(II)-CN-Ti(IV)(particle). This finding suggests that the electron injection is to either an individual titanium surface site or a small number of Ti centers localized around the point of ferrocyanide coordination to the particle and not into a conduction band orbital delocalized over the nanoparticle. The polarizability change, Tr(Deltaalpha), between the ground and the excited states of the Fe(II)(CN)(6)(4)(-)-TiO(2)(particle) system is approximately 3 time larger than normally observed in mixed-valence dinuclear metal complexes. It is proposed that the large polarizability of the excited state increases the dipole-moment changes measured by Stark spectroscopy.     

Density functional calculations of potential energy surface and charge transfer integrals in molecular triphenylene derivative HAT6

We investigate the effect of structural fluctuations on charge transfer integrals, overlap integrals, and site energies in a system of two stacked molecular 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT₆), which is a model system for conducting devices in organic photocell applications. A density functional based computational study is reported. Accurate potential energy surface calculations are carried out using an improved meta-hybrid density functional to determine the most stable configuration of the two weakly bound HAT₆ molecules. The equilibrium parameters in terms of the twist angle and co-facial separation are calculated. Adopting the fragment approach within the Kohn–Sham density functional framework, these parameters are combined to a lateral slide, to mimic structural/conformational fluctuations and variations in the columnar phase. The charge transfer and spatial overlap integrals, and site energies, which form the matrix element of the Kohn–Sham Hamiltonian are derived. It is found that these quantities are strongly affected by the conformational variations. The spatial overlap between stacked molecules is found to be of considerable importance since charge transfer integrals obtained using the fragment approach differ significantly from those using the dimer approach.     

(EDT-TTF-CONH2)6[Re6Se8(CN)6], a metallic Kagome-type organic-inorganic hybrid compound: electronic instability, molecular motion, and charge localization

(EDT-TTF-CONH2)6[Re6Se8(CN)6], space group R, was prepared by electrocrystallization from the primary amide-functionalized ethylenedithiotetrathiafulvalene, EDT-TTF-CONH2 (E(1/2)1 = 0.49 V vs SCE in CH3CN), and the molecular cluster tetraanion, [Re6Se8(CN)6]4- (E(1/2) = 0.33 V vs SCE in CH3CN), equipped with hydrogen bond donor and hydrogen bond acceptor functionalities, respectively. Its Kagome topology is unprecedented for any TTF-based materials. The metallic state observed at room temperature has a strong two-dimensional character, in coherence with the Kagome lattice symmetry, and the presence of minute amounts of [Re6Se8(CN)6](3-)* identified by electron spin spectroscopy. A structural instability toward a distorted form of the Kagome topology of lesser symmetry is observed at ca. 180 K. The low-temperature structure is associated with a localized, electrically insulating electronic ground state and its magnetic susceptibility accounted for by a model of uniform chains of localized S = 1/2 spins in agreement with the 100 K triclinic crystal structure and band structure calculations. A sliding motion, within one out of the three (EDT-TTF-CONH2)2 dimers coupled to the [Re6Se8(CN6)(3-)*]/[Re6Se8(CN6)4-] proportion at any temperature, and the electronic ground state of the organic-inorganic hybrid material are analyzed on the basis of ESR, dc conductivity, 1H spin-lattice relaxation, and static susceptibility data which qualify a Mott localization in [EDT-TTF-CONH2]6[Re6Se8(CN)6]. The coupling between the metal-insulator transition and a structural transition allows for the lifting of a degeneracy due to the ternary axis in the high temperature, strongly correlated metallic phase which, in turn, leads to Heisenberg chains at low temperature.     

Interaction of charge transfer excitons with phonons in molecular crystals

Interaction of charge transfer excitons with phonons due to modulation of the Coulomb electron-hole attraction by lattice oscillations is considered. Such interaction leads in some cases to self-trapping of CT excitons and, as a result, to the hopping character of their migration in the crystal. Spectra of optical absorption with the excitation of these states take the form of broad gaussian curves.     

Surface transfer doping of hydrogen-terminated diamond by C60F48: energy level scheme and doping efficiency

Surface sensitive C1s core level photoelectron spectroscopy was used to examine the electronic properties of C(60)F(48) molecules on the C(100):H surface. An upward band bending of 0.74 eV in response to surface transfer doping by fluorofullerene molecules is measured. Two distinct molecular charge states of C(60)F(48) are identified and their relative concentration determined as a function of coverage. One corresponds to ionized molecules that participate in surface charge transfer and the other to neutral molecules that do not. The position of the lowest unoccupied molecular orbital of neutral C(60)F(48) which is the relevant acceptor level for transfer doping lies initially 0.6 eV below the valence band maximum and shifts upwards in the course of transfer doping by up to 0.43 eV due to a doping induced surface dipole. This upward shift in conjunction with the band bending determines the occupation of the acceptor level and limits the ultimately achievable hole concentration with C(60)F(48) as a surface acceptor to values close to 10(13) cm(-2) as reported in the literature.     

Surface charge tuning of ceria particles by titanium doping: Towards significantly improved polishing performance

Titanium-doped ceria Ce_1 ? x Ti_xO₂ (x = 0–0.3) powders were prepared and their material removal rate (MRR) values for polishing the ZF₇ optical glass were evaluated with respect to their particle sizes, surface charges, crystallinity as well as the suspension stability. Significantly increased MRR values with a particle zeta potential dependence were observed for all the Ti-doped ceria powders, indicating that ceria abrasives with high MRR can be designed and synthesized by tuning particle surface charge using the titanium doping method. The XRD and Raman spectroscopic analyses revealed that the large increase in MRR and the surface negative zeta potentials were attributed to lattice defects due to the formation of CeO₂–TiO₂ solid solutions and the CeTi₂O₆ phase. A maximum MRR value of 544 nm min^?1 was obtained using Ce_0.9Ti_0.1O₂ solid solution as a polishing powder for the ZF7 glass. This value is ca. 2.2 times of that obtained from using pure ceria. With the x value further increasing to 0.2 and 0.3, the MRR value decreased slightly with the CeTi₂O₆ phase content increasing. This fact reveals that the contribution of CeTi₂O₆ to the MRR increase is less than that of CeO₂–TiO₂ solid solution.     

Modulation of the thermodynamic, kinetic, and magnetic properties of the hydrogen monomer on graphene by charge doping

The thermodynamic, kinetic, and magnetic properties of the hydrogen monomer on doped graphene layers were studied by ab initio simulations. Electron doping heightens the diffusion potential barrier, while hole doping lowers it. However, both kinds of dopings heighten the desorption potential barrier. The underlying mechanism was revealed by investigating the effect of charge doping on the bond strength of graphene and on the electron transfer and the coulomb interaction between the hydrogen monomer and graphene. The kinetic properties of H and D monomers on doped graphene layers during both the annealing process (annealing time t(0) = 300 s) and the constant-rate heating process (heating rate α = 1.0 K/s) were simulated. Macroscopic diffusion of hydrogen monomers on graphene can be achieved when the doping-hole density reaches 5.0 × 10(13) cm(-2). Both electron and hole dopings linearly reduce the total magnetic moment and exchange splitting, which was explained by a simple exchange model. The laws found in this work had been generalized to explain many phenomena reported in literature. This study can further enhance the understanding of the interaction between hydrogen and graphene and was expected to be helpful in the design of hydrogenated-graphene-based devices.     

Heptamethine cyanine dyes with a large stokes shift and strong fluorescence: a paradigm for excited-state intramolecular charge transfer

New heptamethine cyanine dyes with an alkylamino group at the central position were found to exhibit a large Stokes shift (>140 nm) and strong fluorescence. They were suggested to be a new paradigm for excited-state intramolecular charge transfer (ICT). The configuration change of the bridgehead amine accompanying ICT was investigated in different viscosity and pH media.