Dongdong Zhang

Bio: Dongdong Zhang is an academic researcher from Peking University. The author has contributed to research in topics: Graphene & Graphene nanoribbons. The author has an hindex of 4, co-authored 5 publications receiving 182 citations. Previous affiliations of Dongdong Zhang include Center for Excellence in Education & Chinese Academy of Sciences.

Control of carrier type and density in exfoliated graphene by interface engineering.

Abstract: Air-stable, n-doped or p-doped graphene sheets on a chip were achieved by modifying the substrates with self-assembled layers of silane and polymer. The interfacial effects on the electronic properties of graphene were investigated using micro-Raman and Kelvin probe force microscopy (KPFM). Raman studies demonstrated that the phonon vibrations were sensitive to the doping level of graphene on the various substrates. Complementary information on the charge transfer between the graphene and substrate was extracted by measuring the surface potential of graphene flakes using KPFM, which illustrated the distribution of carriers in different graphene layers as well as the formation of dipoles at the interface. The Fermi level of single layer graphene on the modified substrates could be tuned in a range from −130 to 90 mV with respect to the Dirac point, corresponding to the doped carrier concentrations up to 1012 cm−2.

Nanoscale charge distribution and energy band modification in defect-patterned graphene

Abstract: Defects were introduced precisely to exfoliated graphene (G) sheets on a SiO(2)/n(+) Si substrate to modulate the local energy band structure and the electron pathway using solution-phase oxidation followed by thermal reduction. The resulting nanoscale charge distribution and band gap modification were investigated by electrostatic force microscopy and spectroscopy. A transition phase with coexisting submicron-sized metallic and insulating regions in the moderately oxidized monolayer graphene were visualized and measured directly. It was determined that the delocalization of electrons/holes in a graphene "island" is confined by the surrounding defective C-O matrix, which acts as an energy barrier for mobile charge carriers. In contrast to the irreversible structural variations caused by the oxidation process, the electrical properties of graphene can be restored by annealing. The defect-patterned graphene and graphene oxide heterojunctions were further characterized by electrical transport measurement.

Optical Spectroscopy Investigation of the Structural and Electrical Evolution of Controllably Oxidized Graphene by a Solution Method

Abstract: A precisely controlled chemical modification of exfoliated graphene on a substrate was achieved by solution-phase oxidation. The structural and electrical evolution of graphene induced by oxygen-related defects was investigated using micro-Raman and photoluminescence spectroscopy. The sp2-hybrid carbon network in monolayer graphene was found to gradually decrease with increasing degree of oxidation. The size of the graphene quantum dots was finally reduced to about 1 nm, which exhibited an energy band gap of 2 eV. The double-layer graphene showed a symmetry breaking induced by the defects. The process of solution modification may provide a facile method to tailor the electrical properties of graphene on a chip for constructing carbon-based nanoelectronics.

Device for measuring subsurface structure characteristic and micro-area wideband dielectric property

Abstract: The invention discloses a device for measuring subsurface structure characteristic and micro-area wideband dielectric property, and the device comprises an electrostatic force microscope which comprises a conductive miniature cantilever probe comprising a conductive probe part and a miniature cantilever probe part, wherein a probe carries out the first scanning of a to-be-measured sample in a touch scanning mode. The device also comprises a phase-locking amplifier which is used for receiving a reference frequency signal and a real-time signal generated in the first scanning process, and generating a first phase signal according to the reference frequency signal and the real-time signal. After the first scanning, the probe carries out the second scanning of the to-be-measured sample along the scanning trace of the first scanning in a mode of lifted scanning. The phase-locking amplifier also generates a second phase signal according to the reference frequency signal and a real-time signal generated in the second scanning process. The device also comprises a signal source which enables a modulation voltage signal to be applied to the conductive probe part in the second scanning process, and to be outputted to an external phase-locking amplifier. The device also comprises the external phase-locking amplifier which is used for receiving the modulation voltage signal and the second phase signal, and obtaining a dielectric response signal and a dielectric loss angle signal according to the modulation voltage signal and the second phase signal.

Manipulation of electron transport in graphene by nanopatterned electrostatic potential on an electret

Abstract: The electron transport characteristics of graphene can be finely tuned using local electrostatic fields. Here, we use a scanning probe technique to construct a statically charged electret gate that enables in-situ fabrication of graphene devices with precisely designed potential landscapes, including p-type and n-type unipolar graphene transistors and p-n junctions. Electron dynamic simulation suggests that electron beam collimation and focusing in graphene can be achieved via periodic charge lines and concentric charge circles. This approach to spatially manipulating carrier density distribution may offer an efficient way to investigate the novel electronic properties of graphene and other low-dimensional materials.

Selective ionic transport through tunable subnanometer pores in single-layer graphene membranes.

Abstract: We report selective ionic transport through controlled, high-density, subnanometer diameter pores in macroscopic single-layer graphene membranes. Isolated, reactive defects were first introduced into the graphene lattice through ion bombardment and subsequently enlarged by oxidative etching into permeable pores with diameters of 0.40 ± 0.24 nm and densities exceeding 1012 cm–2, while retaining structural integrity of the graphene. Transport measurements across ion-irradiated graphene membranes subjected to in situ etching revealed that the created pores were cation-selective at short oxidation times, consistent with electrostatic repulsion from negatively charged functional groups terminating the pore edges. At longer oxidation times, the pores allowed transport of salt but prevented the transport of a larger organic molecule, indicative of steric size exclusion. The ability to tune the selectivity of graphene through controlled generation of subnanometer pores addresses a significant challenge in the dev...

Single-Gate Bandgap Opening of Bilayer Graphene by Dual Molecular Doping

Abstract: Dual doping-driven perpendicular electric field with opposite directions remarkably increase the on/off current ratio of bilayer graphene field-effect transistors. This unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH2 -functionalized self-assembled monolayers (SAMs)) even in a single-gate device structure.

Graphene-P(VDF-TrFE) multilayer film for flexible applications.

Abstract: A flexible, transparent acoustic actuator and nanogenerator based on graphene/P(VDF-TrFE)/graphene multilayer film is demonstrated. P(VDF-TrFE) is used as an effective doping layer for graphene and contributes significantly to decreasing the sheet resistance of graphene to 188 ohm/sq. The potentiality of graphene/P(VDF-TrFE)/graphene multilayer film is realized in fabricating transparent, flexible acoustic devices and nanogenerators to represent its functionality. The acoustic actuator shows good performance and sensitivity over a broad range of frequency. The output voltage and the current density of the nanogenerator are estimated to be ∼3 V and ∼0.37 μAcm–2, respectively, upon the application of pressure. These values are comparable to those reported earlier for ZnO- and PZT-based nanogenerators. Finally, the possibility of rollable devices based on graphene/P(VDF-TrFE)/graphene structure is also demonstrated under a dynamic mechanical loading condition.

Carrier control of MoS2 nanoflakes by functional self-assembled monolayers.

Abstract: Carrier doping of MoS2nanoflakes wasachieved by functionalself-assembledmonolayers (SAMs) withdifferentdipolemoments.Theeffectof SAMson thecharge transferbetweenthesubstratesandMoS2nanoflakeswasstudiedbyRamanspectroscopy, field-effecttransistor(FET) measurements,andKelvinprobemicroscope (KFM).RamandataandFETresultsverifiedthat fluoroalkyltrichlorosilane-SAMwithalargepositivedipolemoment,actingasholedonors,significantlyreducedthe intrinsic n-doping characteristic of MoS2 nanoflakes, while 3-(trimethoxysilyl)-1-propanamine-SAMs, acting as electron donors, enhanced the n-doping characteristic. The additional built-in electric field at the interface between SiO2 substrates and MoS2 nanoflakes induced by SAMs with molecular dipole moments determined the charge transfer process. KFM results clearly demonstrated the charge transfer between MoS2 and SAMs and the obvious interlayer screening effect of the pristine and SAM-modified MoS2nanoflakes. However, the KFM results were not fully consistent with the Raman and FET results since the externally absorbed water molecules were shown to partially shield the actual surface potential measurement. By eliminating the contribution of th ew ater molecules,theFermilevelofmonolayerMoS2couldbeestimatedtomodulateinarangeofmorethan0.45� 0.47eV.Thisworkmanifeststhattheworkfunctionof MoS2 nanoflakes can be significantly tuned by SAMs by virtue of affecting the electrostatic potential between the substrates and MoS2 nanoflakes.