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Electrochemical Polishing of Two-Dimensional Materials
摘要： Two-dimensional (2D) layered materials demonstrate their exquisite properties such as high temperature superconductivity, superlubricity, charge density wave, piezotronics, flextronics, straintronics, spintronics, valleytronics, and optoelectronics, mostly, at the monolayer limit. Following initial breakthroughs based on micromechanically exfoliated 2D monolayers, significant progress has been made in recent years towards the bottom-up synthesis of large-area monolayer 2D materials such as MoS2 and WS2 using physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques in order to facilitate their transition into commercial technologies. However, the nucleation and subsequent growth of the secondary, tertiary, and greater numbers of vertical layers poses a significant challenge not only towards the realization of uniform monolayers, but also to maintain their consistent electronic and optoelectronic properties which change abruptly when transitioning from the monolayer to multilayer form. Chemical or physical techniques which can remove the unwarranted top layers without compromising the material quality will have tremendous consequence towards the development of atomically flat, large-area, uniform monolayers of 2D materials. Here, we report a simple, elegant, and self-limiting electrochemical polishing technique which can thin down any arbitrary thickness of 2D material, irrespective of whether these are obtained using powder vapor transport (PVT) or mechanical exfoliation, into their corresponding monolayer form at room temperature within a few seconds without compromising their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D transition metal dichalcogenides (TMDCs) owing to the stability of their basal planes, enhanced edge reactivity, and stronger-than-van der Waals (vdW) interaction with the substrate. Our study also reveals that 2D monolayers are chemically more robust and corrosion resistant compared to their bulk counterparts in similar oxidative environments which enables electrochemical polishing of such materials down to a monolayer.
关键词： physical vapor transport,MoS2,WS2,monolayer,corrosion,two-dimensional (2D) materials,electrochemical polishing,electro-ablation
更新于2025-09-23 15:21:21
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Electronic transport in degenerate (100) scandium nitride thin films on magnesium oxide substrates
摘要： Scandium nitride (ScN) is a degenerate n-type semiconductor with very high carrier concentrations, low resistivity, and carrier mobilities comparable to those of transparent conducting oxides such as zinc oxide. Because of its small lattice mismatch to gallium nitride (GaN), <1%, ScN is considered a very promising material for future GaN based electronics. Impurities are the source of the degeneracy. Yet, which specific impurities are the cause has remained in contention. ScN thin films of various thicknesses were grown on magnesium oxide substrates in a (001) orientation using reactive magnetron sputtering across a range of deposition conditions. X-ray diffraction was used to verify crystal orientation. Film thicknesses ranging from 39 to 85 nm were measured using scanning electron microscopy. The electronic transport properties of the films were characterized using Hall-effect measurements at temperatures ranging from 10 to 320 K. At 10 K, the electron concentration varies from 4.4 (cid:2) 1020 to 1.5 (cid:2) 1021 cm(cid:3)3, resistivity from 2.1 (cid:2) 10(cid:3)4 to 5.0 (cid:2) 10(cid:3)5 X(cid:4)cm, and Hall mobility from 66 to 97 cm2/V(cid:4)s. Secondary ion mass spectroscopy (SIMS) was used to determine film compositions. Finally, density functional theory (DFT) was used to compute the activation energies for various point defects including nitrogen and scandium vacancies and oxygen and fluorine substituting for nitrogen. For both oxygen and fluorine substitution, the energies were negative, indicating spontaneous formation. Nevertheless, the combined results of the Hall, SIMS, and DFT strongly suggest that oxygen substitution is the primary mechanism behind the high carrier concentration in these samples.
关键词： degenerate n-type semiconductor,Hall-effect measurements,Scandium nitride,density functional theory,electronic transport properties
更新于2025-09-23 15:21:21
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Electronic properties of graphene with point defects
摘要： This article attempts a point-by-point review of the electron spectrum of graphene containing defects (adsorbed atoms, substitutional atoms, vacancies) that can be adequately described within the Lifshitz model. In this regard, the known Hamiltonian for this model is calculated for the case of two-dimensional relativistic electrons, and the criteria for occurrence of an impurity resonance near a Dirac point are given. The theory of concentration band structure transformation of graphene is presented, from which it follows that a transport gap is opened upon reaching a strictly defined value of impurity concentration in the neighborhood of the energy of impurity resonance. At the same time, the question of the possibility (or impossibility) of localization in such a {spatially disordered system} of Dirac quasiparticles is analyzed. Based on this, it is possible to provide an explanation and present a description of a phenomenon recently observed in {impure graphene}, the metal-dielectric transition that appears as a direct consequence of the decrease of the Fermi energy of the system in the region of a transport gap. The concept of local spectrum rearrangement of graphene, which also occurs during the process of increased concentration of defects in it, is introduced and substantiated. Physical reasons are formulated, by which the position of the minimum of the low-temperature conductivity of graphene as a function of the Fermi energy of electrons corresponds to the impurity resonance energy rather than to the Dirac point, as has been validated in a number of theoretical and test studies. Here, it appears that the minimum value is not a universal magnitude, but depends on the concentration of defects. Analytical examination of impurity effects is accompanied by numerical modeling of the system under study, as a result of which complete correspondence has been established between these two approaches. In particular, the overall picture of spectrum rearrangement, localization of electron states, and also the effects having local nature are confirmed.
关键词： electron spectrum,transport gap,Lifshitz model,impurity resonance,metal-dielectric transition,graphene,local spectrum rearrangement,conductivity,point defects
更新于2025-09-23 15:21:21
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-type PbTe from first principles
摘要： We present an ab initio study that identifies the main electron-phonon scattering channels in n-type PbTe. We develop an electronic transport model based on the Boltzmann transport equation within the transport relaxation time approximation, fully parametrized from first-principles calculations that accurately describe the dispersion of the electronic bands near the band gap. Our computed electronic mobility as a function of temperature and carrier concentration is in good agreement with experiments. We show that longitudinal optical phonon scattering dominates electronic transport in n-type PbTe, while acoustic phonon scattering is relatively weak. We find that scattering due to soft transverse optical phonons is by far the weakest scattering mechanism, due to the symmetry-forbidden scattering between the conduction band minima and the zone center soft modes. Soft phonons thus play the key role in the high thermoelectric figure of merit of n-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity. Our results suggest that materials like PbTe with soft modes that are weakly coupled with the electronic states relevant for transport may be promising candidates for efficient thermoelectric materials.
关键词： electron-phonon scattering,transport relaxation time approximation,thermoelectric materials,Boltzmann transport equation,n-type PbTe
更新于2025-09-23 15:21:01
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[IEEE 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Chicago, IL, USA (2019.6.16-2019.6.21)] 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Room Temperature Processed Transparent Cu-Zn-S Nanocomposites as Hole Transport Materials in CdTe Photovoltaics
摘要： Here, we report room temperature processed Cu-Zn-S ternary thin films fabricated using SILAR method as a back-contact hole transport layer in cadmium telluride (CdTe) solar cells. These Cu-Zn-S films are transparent to visible region with compact grains, and high conductivity. X-ray diffraction (XRD) measurements shows the crystalline nature of the as-deposited Cu-Zn-S films. The Cu-Zn-S nanocomposite as a back contact buffer layer in CdTe devices improves the device performance to 12.7% (average 12.4%) from 10.4% (average 9.8%) compared to a Au only back contact and is comparable to Cu/Au back contact (thermally evaporated). The temperature dependence current voltage characteristics shows the reduced back barrier height compared to Au only and Cu/Au back contact.
关键词： SILAR,Cu-Zn-S,back contact,hole transport layer (HTL),solar cells
更新于2025-09-23 15:21:01
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Hydroxymethyl Functionalized PEDOT-MeOH:PSS for Perovskite Solar Cells
摘要： Poly(hydroxymethylated-3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT-MeOH:PSS) conducting polymers are synthesized and incorporated in inverted structured perovskite solar cells (PVSCs) as hole transport materials. The highest occupied molecular orbital of PEDOT-MeOH is lowered by adding a hydroxymethyl (-MeOH) functional group to ethylenedioxythiophene (EDOT), and thus the work function of PEDOT-MeOH:PSS is increased. Additionally, hydrogen bonding can be formed among EDOT-MeOH monomers and between EDOT-MeOH monomers and sulfate groups on PSS, which promote PEDOT-MeOH chain growth and enhance PSS doping. The electronic, microstructural, and surface morphological properties of PEDOT-MeOH:PSS are modified by changing the amount of PSS and ferric oxidizing agent used in the polymerization and by adding ethylene glycol in the post-synthesis treatment. The PVSCs based on ethylene glycol treated PEDOT-MeOH:PSS overperform the PVSCs based on commercial PEDOT:PSS because of the better energetic alignment and the enhancement of PEDOT-MeOH:PSS electrical conductivity. This work opens the way to develop new hole transport materials for highly efficient inverted PVSCs.
关键词： hole transport material,perovskite,solar cell,electrical conductivity,work function
更新于2025-09-23 15:21:01
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Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods
摘要： Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical arrays of nanorods. The ability to define the morphology and structure of the nanorods is essential to maximising the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show the O-polar nanorods (~0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (~0.37 eV). Oxygen plasma treatment is shown by Cathodoluminescence (CL) spectroscopy to reduce mid-gap defects associated with radiative emissions that improves the Schottky contacts from weakly-rectifying to strongly-rectifying. Interestingly, the plasma treatment was shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc vacancy related hydrogen defect complexes (VZn, Hn) in Zn-polar nanorods that evolve to lower coordinated complexes. The effect on HO in the O-polar nanorods coincided with a large reduction in the visible range defects producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled enhancing devices such as dynamic photodetectors, strain sensors, and LEDs while showing the O-polar nanorods can outperform Zn-polar nanorods in such applications.
关键词： ZnO,Cathodoluminescence,Electrical Transport,Polarity,Schottky Contacts,Nanorods,Defects
更新于2025-09-23 15:21:01
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D-??-D molecular semiconductors for perovskite solar cells: the superior role of helical versus planar ??-linker
摘要： Controlling the mode of molecular packing and the size of molecular aggregate are of fundamental importance for high-performance charge transporting materials in next-generation optoelectronic devices. To clarify the peculiar role of helicene as kernal blocks in the exploration of unconventional organic semiconductors, in this work thia[5]helicene (T5H) is duplicately aminated with electron-donating dimethoxydiphenylamine to afford T5H-OMeDPA, which is systematically compared with its perylothiophene (PET) congener (PET-OMeDPA). On the basis of quantum theory of atoms in molecules and energy decomposition analysis of single-crystals, it is suprisingly found that while π-π stacking of planar PET is stronger than that of helical T5H, this desirable effect for charge transport of organic semiconductors is completely lost for donor-π-donor (D-π-D) typed PET-OMeDPA, but is retained for T5H-OMeDPA to a large extent. Consequently, T5H-OMeDPA single-crystal presents an about 5 times higher theoretical hole-mobility than PET-OMeDPA. More critically, the solution-processed racemic glassy film of T5H-OMeDPA displays a 3 times higher hole-mobility in comparison with the PET-OMeDPA counterpart, due to a larger domain of molecular aggregate. With respect to PET-OMeDPA, there is a weaker electronic coupling of helical T5H-OMeDPA with perovskite, leading to a reduced interfacial charge recombination. Due to reduced transport resistance and enhanced recombination resistance, perovskite solar cells with T5H-OMeDPA exhibit a power conversion efficiency of 21.1%, higher than that of 19.8% with PET-OMeDPA and that of 20.6% with spiro-OMeTAD control.
关键词： charge transport,perovskite solar cells,helicene,molecular semiconductors,π-π stacking
更新于2025-09-23 15:21:01
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Anthradithiophene based hole-transport material for efficient and stable perovskite solar cells
摘要： A novel hole-transport material (HTM) based on an anthradithiophene central bridge named BTPA-7 is developed. In comparison to spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene), the synthetic steps of BTPA-7 are greatly reduced from 6 to 3 and the synthetic cost of BTPA-7 is nearly a half that of spiro-OMeTAD. Moreover, BTPA-7 exhibits a relatively lower conductivity but higher hole mobility and higher glass transition temperature (Tg) than spiro-OMeTAD. Compared with the photovolatic performance for spiro-OMeTAD, FA0.85MA0.15PbI3 and MAPbI3 PSC devices based on BTPA-7 exhibit slightly lower PCEs with the values of 17.58% (18.88% for spiro-OMeTAD) and 11.90% (13.25% for spiro-OMeTAD), respectively. Nevertheless, a dramatically higher Jsc of PSC based on BTPA-7 is achieved, which arises from the higher hole mobility of BTPA-7. In addition, the relatively hydrophobic character of BTPA-7 eventually enhances the PSC device stability. Lower cost, higher hole mobility, higher Tg, satisfactory photovoltaic performance, and superior device stability of BTPA-7 can be utilized as a substitute for spiro-OMeTAD in PSCs.
关键词： Stability,Anthradithiophene,Hole-transport material,Synthetic cost
更新于2025-09-23 15:21:01
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Enhanced stability and efficiency in inverted perovskite solar cells through graphene doping of PEDOT:PSS hole transport layer
摘要： Poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) plays a relevant role in the device performance as hole extraction layer (HTL) of inverted perovskite solar cells. Here, we show a simple low-temperature spin coating method for obtaining homogenous graphene-doped thin films of PEDOT:PSS with improved electrical conductivity without decreasing optical transmittance. Moreover, the crystallinity and stability in ambient conditions of the perovskite grown on it are enhanced. The hydrophobic character of graphene probably blocks undesirable reactions hampering degradation. By impedance spectroscopy it is demonstrated better charge extraction and reduction of recombination mechanisms at the doped-HTL/perovskite interface, resulting in improved photovoltaic parameters of the solar cell and greater stability at room operation conditions thus providing a simple and cost-effective method of preparing solar cells based on hybrid perovskites.
关键词： perovskite solar cell,PEDOT:PSS,doping,graphene,hole transport layer,impedance spectroscopy
更新于2025-09-23 15:21:01