arXiv subjects

Zhenyu Li

Publications and source records attributed to Zhenyu Li.

At least 181 records · Page 10Linked to original sources

arXiv2011

Chemical Vapor Deposition Growth of Graphene using Other Hydrocarbon Sources

Graphene has attracted a lot of research interests due to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000°C, is required for such growth. Here we demonstrate a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400°C. When benzene is used as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300°C. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to undemanding route for economical and convenient graphene growth.

cond-mat.mtrl-sci↗

arXiv2011

Structure of graphene oxide: thermodynamics versus kinetics

Graphene oxide (GO) is an important intermediate to prepare graphene and it is also a versatile material with various applications. However, despite its importance, the detailed structure of GO is still unclear. For example, previous theoretical studies based on energetics have suggested that hydroxyl chain is an important structural motif of GO, which, however, is found to be contrary to nuclear magnetic resonance (NMR) experiment. In this study, we check both thermodynamic and kinetic aspects missed previously. First principles thermodynamics gives a free energy based stability ordering similar to that based on energetics, and hydroxyl chain is thus thermodynamically still favorable. At the same time, by checking the calculated vibrational frequencies, we note that hydroxyl chain structure is also inconsistent with infrared experiment. Therefore, kinetics during GO synthesis is expected to make an important role in GO structure. Transition state calculations predict large energy barriers between local minima, which suggests that experimentally obtained GO has a kinetically constrained structure.

cond-mat.mtrl-sci↗

arXiv2010

Nearly Free Electron States in Graphane Nanoribbon Superlattice

Nearly free electron (NFE) state has been widely studied in low dimensional systems. Based on first-principles calculations, we identify two types of NFE states in graphane nanoribbon superlattice, similar to those of graphene nanoribbons and boron nitride nanoribbons. Effect of electron doping on the NFE states in graphane nanoribbon superlattice has been studied, and it is possible to open a vacuum transport channel via electron doping.

cond-mat.mtrl-sci↗

arXiv2010

Nearly Free Electron States in Graphene Nanoribbon Superlattices

Nearly free electron (NFE) state is an important kind of unoccupied state in low dimensional systems. Although it is intensively studied, a clear picture on its physical origin and its response behavior to external perturbations is still not available. Our systematic first-principles study based on graphene nanoribbon superlattices suggests that there are actually two kinds of NFE states, which can be understood by a simple Kronig-Penney potential model. An atom-scattering-free NFE transport channel can be obtained via electron doping, which may be used as a conceptually new field effect transistor.

cond-mat.mtrl-sci↗

arXiv2010

Coalescence of Carbon Atoms on Cu (111) Surface: Emergence of a Stable Bridging-Metal Structure Motif

By combining first principles transition state location and molecular dynamics simulation, we unambiguously identify a carbon atom approaching induced bridging metal structure formation on Cu (111) surface, which strongly modify the carbon atom coalescence dynamics. The emergence of this new structural motif turns out to be a result of the subtle balance between Cu-C and Cu-Cu interactions. Based on this picture, a simple theoretical model is proposed, which describes a variety of surface chemistries very well.

cond-mat.mtrl-sci↗

arXiv2010

First Principles NMR Signatures of Graphene Oxide

Nuclear magnetic resonance (NMR) has been widely used in the graphene oxide (GO) structure study. However, the detailed relationship between its spectroscopic features and the GO structural configuration has remained elusive. Based on first principles $^{13}$C chemical shift calculations using the gauge including projector augmented waves (GIPAW) method, we provide a spectrum-structure connection. Chemical shift of carbon is found to be very sensitive to atomic environment, even with an identical oxidation group. Factors determining the chemical shifts for epoxy and hydroxy groups have been discussed. GO structures previously reported in the literature have been checked from the NMR point of view. The energetically favorable hydroxy chain structure is not expected to be widely existed in real GO samples according to our NMR simulations. The epoxy pair we proposed previously is also supported by chemical shift calculations.

cond-mat.mtrl-sci↗

arXiv2009

How Graphene Is Cut upon Oxidation?

Our first principles calculations reveal that oxidative cut of graphene is realized by forming epoxy and then carbonyl pairs. Direct forming carbonyl pair to tear graphene up from an edge position is not favorable in energy. This atomic picture is valuable for developing effective graphene manipulation means. The proposed epoxy pairs may be related to some long puzzling experimental observations on graphene oxide.

cond-mat.mtrl-sci↗

arXiv2009

Oxidation States of Graphene: Insights from Computational Spectroscopy

When it is oxidized, graphite can be easily exfoliated forming graphene oxide (GO). GO is a critical intermediate for massive production of graphene, and it is also an important material with various application potentials. With many different oxidation species randomly distributed on the basal plane, GO has a complicated nonstoichiometric atomic structure that is still not well understood in spite of of intensive studies involving many experimental techniques. Controversies often exist in experimental data interpretation. We report here a first principles study on binding energy of carbon 1s orbital in GO. The calculated results can be well used to interpret experimental X-ray photoelectron spectroscopy (XPS) data and provide a unified spectral assignment. Based on the first principles understanding of XPS, a GO structure model containing new oxidation species epoxy pair and epoxy-hydroxy pair is proposed. Our results demonstrate that first principles computational spectroscopy provides a powerful means to investigate GO structure.

cond-mat.mtrl-sci↗

arXiv2009

Chemical functionalization on planar polysilane and graphane

Two dimensional materials are important for electronics applications. A natural way for electronic structure engineering on two dimensional systems is on-plane chemical functionalization. Based on density functional theory, we study the electronic structures of fluorine substituted planar polysilane and graphane. We find that carbon and silicon present very different surface chemistry. The indirect energy gap of planar polysilane turns to be direct upon fluorine decoration, and the gap width is mainly determined by fluorine coverage regardless of its distribution on the surface. However, electronic structure of fluorine doped graphane is very sensitive to the doping configuration, due to the competition between antibonding states and nearly-free-electron (NFE) states. With specific fluorine distribution pattern, zero-dimensional and one-dimensional NFE states can be obtained. We have also studied the chemical modification with -OH or -NH$_2$ group. Carbon seems to be too small to accommodate big functional groups on its graphane skeleton with a high concentration.

cond-mat.mtrl-sci↗

arXiv2009

A first principles study on FeAs single layers

FeAs- single layer is tested as a simple model for LaFeAsO and BaFe2As2 based on first-principles calculations using generalized gradient approximation (GGA) and GGA+U. The calculated single- layer geometric and electronic structures are inconsistent with that of bulk materials. The bulk collinear antiferromagnetic ground state is failed to be obtained in the FeAs- single layer. The monotonous behavior of the Fe-As distance in z direction upon electron or hole doping is also in contrast with bulk materials. Our results indicate that, in LaFeAsO and BaFe2As2, interactions between FeAs layer and other layers beyond simple charge doping are important, and a single FeAs layer may not represent a good model for Fe based superconducting materials.

cond-mat.supr-con↗

arXiv2008

A first principles study on the electronic and magnetic properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$

We report a systematic first-principles study on the recent discovered superconducting Ba$_{1-x}$K$_x$Fe$_2$As$_2$ systems ($x$ = 0.00, 0.25, 0.50, 0.75, and 1.00). Previous theoretical studies strongly overestimated the magnetic moment on Fe of the parent compound BaFe$_2$As$_2$. Using a negative on-site energy $U$, we obtain a magnetic moment 0.83 $μ_B$ per Fe, which agrees well with the experimental value (0.87 $μ_B$). K doping tends to increase the density of states at fermi level. The magnetic instability is enhanced with light doping, and is then weaken by increasing the doping level. The energetics for the different K doping sites are also discussed.

cond-mat.supr-con↗

arXiv2008

Electronic structures of organic molecule encapsulated BN nanotubes under transverse electric field

The electronic structures of boron nitride nanotubes (BNNTs) doped by different organic molecules under a transverse electric field were investigated via first-principles calculations. The external field reduces the energy gap of BNNT, thus makes the molecular bands closer to the BNNT band edges and enhances the charge transfers between BNNT and molecules. The effects of the electric field direction on the band structure are negligible. The electric field shielding effect of BNNT to the inside organic molecules is discussed. Organic molecule doping strongly modifies the optical property of BNNT, and the absorption edge is red-shifted under static transverse electric field.

cond-mat.mtrl-sci↗

arXiv2008

Magnetism in Graphene Systems

Graphene has attracted a great interest in material science due to its novel electronic structrues. Recently, magnetism discovered in graphene based systems opens the possibility of their spintronics application. This paper provides a comprehensive review on the magnetic behaviors and electronic structures of graphene systems, including 2-dimensional graphene, 1-dimensional graphene nanoribbons, and 0-dimensional graphene nanoclusters. Theoretical research suggests that such metal-free magnetism mainly comes from the localized states or edges states. By applying external electric field, or by chemical modification, we can turn the zigzag nanoribbon systems to half metal, thus obtain a perfect spin filter.

cond-mat.mtrl-sci↗

arXiv2008

Electronic Structures of SiC Nanoribbons

Electronic structures of SiC nanoribbons have been studied by spin-polarized density functional calculations. The armchair nanoribbons are nonmagnetic semiconductor, while the zigzag nanoribbons are magnetic metal. The spin polarization in zigzag SiC nanoribbons is originated from the unpaired electrons localized on the ribbon edges. Interestingly, the zigzag nanoribbons narrower than $\sim$4 nm present half-metallic behavior. Without the aid of external field or chemical modification, the metal-free half-metallicity predicted for narrow SiC zigzag nanoribbons opens a facile way for nanomaterial spintronics applications.

cond-mat.mtrl-sci↗

arXiv2008

Electronic structures of defective BN nanotubes under transverse electric fields

We investigate the electronic structures of some defective boron nitride nanotubes (BNNTs) under transverse electric fields within density-functional theory. (16,0) BNNTs with antisite, carbon substitution, single vacancy, and Stone-Wales 5775 defects are studied. Under transverse electric fields, the band gaps of the defective BNNTs are reduced, similar to the pristine ones. The energy levels of the defect states vary with the transverse electric field directions, due to the different electrostatic potential shift at the defect sites induced by the electric fields. Therefore, besides electronic structure and optical property engineering, the transverse electric field can be used to identify the defect positions in BNNTs.

cond-mat.mtrl-sci↗

arXiv2008

A first principles study on organic molecules encapsulated BN nanotubes

The electronic structures of boron nitride nanotubes (BNNTs) doped by organic molecules are investigated with density functional theory. Electrophilic molecule introduces acceptor states in the wide gap of BNNT close to the valence band edge, which makes the doped system a $p$-type semiconductor. However, with typical nucleophilic organic molecules encapsulation, only deep occupied molecular states but no shallow donor states are observed. There is a significant electron transfer from BNNT to electrophilic molecule, while the charge transfer between nucleophilic molecule and BNNT is neglectable. When both electrophilic and nucleophilic molecules are encapsulated in the same BNNT, large charge transfer between the two kinds of molecules occurs. The resulted small energy gap can strongly modify the transport and optical properties of the system.

cond-mat.mtrl-sci↗

arXiv2008

Half Metallicity in Hybrid BCN Nanoribbons

We report a first-principles electronic-structure calculation on C and BN hybrid zigzag nanoribbons. We find that half-metallicity can arise in the hybrid nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. This unexpected half-metallicity in the hybrid nanos-tructures stems from a competition between the charge and spin polarizations, as well as from the pi orbital hybridization between C and BN. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of designing metal-free half metals.

cond-mat.mtrl-sci↗

arXiv2007

Will Zigzag Graphene Nanoribbon Turn to Half Metal under Electric Field?

At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field to induce the half-metallicity decreases with the increase of the ribbon width. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remain half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may leads to some important applications in spinstronics.

cond-mat.mtrl-sci↗