=2019 = 

18.Thermally induced atomic and electronic structure evolution in nanostructured carbon film by in situ TEM/EELS analysis

Applied Surface Science  498 (2019) 143831（PDF-File）

Nan Jiana, Peidong Xue, Dongfeng Diao*

The atomic and electronic structures of nanostructured carbon film are of fundamental importance due to their dominant role in carbon films' physical, chemical and mechanical properties. Here, we reported thermally induced atomic and electronic structure evolution of nanostructured carbon film by in-situ transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Two nanostructured carbon films were synthesized by electron cyclotron resonance (ECR) plasma sputtering under ion and electron irradiation, respectively. We observed that even though the two carbon films had similar initial sp2/sp3 hybridization ratio, they exhibited very different performance during the same heating process. The electron irradiated film only had a relative slight clustering of sp2 carbon crystallites and almost maintained its sp2/sp3 ratio till 1000 °C; while the ion irradiated film had a significant sp2 crystallites clustering, started its sp3-to-sp2 conversion from 200 °C and almost completed at 400 °C. Argon signal was only found in the ion irradiated carbon film and its content reduced simultaneously with the sp3-to-sp2 conversion process. We suggest that the heated argon trapped inside the carbon film were responsible for triggering the sp3-to-sp2 conversion. The result will guide the future application of the nanostructured carbon film in the proper operation temperature range and help to choose appropriate irradiation method for thin film surface modification.

=2017 = 

17. Resolving H(Cl, Br, I) capabilities of transforming solution hydrogen-bond and surface-stress

Chemical Physics Letters 678 (2017) 233–240

Xi Zhang, Yong Zhou, Yinyan Gong, Yongli Huang, Changqing Sun

A combination of differential phonon spectrometrics (DPS) and DFT calculations verified the essentiality of H+-H+ point fragilization and X
polarization dictating the surface stress of HX (X = Cl, Br, I) solutions. H+-H+ repulsion breaks the network regularly; X
polarization shortens and stiffens the H-O bonds but lengthens and softens the O:H nonbonds in its hydration shell. The X
- capability of hydrogen bond and surface stress transformation follows the order of I > Br > Cl. Observations provide fresh insight into the acid solvation network dynamics. DPS resolves solute capabilities of transforming the bonds and surface stress.

16.Physical mechanism of surface roughening on the radial core-shell nanowire heterostructure with alloy shell

AIP Advances 7 (2017), 055006（PDF-File）

Yuanyuan Cao and Dongfeng Diao

We proposed a quantitative thermodynamic theory to address the physical process of surface roughening during the epitaxial growth of core-shellNWwith alloy layer. The surface roughening origins from the transformation of the Frank-van der Merwe (FM) mode to the Stranski-Krastanow (SK) mode. In addition to the radius of NW core, the composition and thickness of alloy shell could determine the growth behaviors due to their modulation to the strain. The established theoretical model not only explains the surface roughening caused by the alloy shell layer, but also provides a new way to control the growth of core-shell NW.

15.Structure and dynamics of water confined in a graphene nanochannel under gigapascal high pressure: dependence of friction on pressure and confinement

Physical Chemistry Chemical Physics 19（2017） , 14048 - 14054（PDF-File）

Lei Yang, Yanjie Guo and Dongfeng Diao

Recently, water flow confined in nanochannels has become an interesting topic due to its unique properties and potential applications in nanofluidic devices. The trapped water is predicted to experience high pressure in the gigapascal regime. Theoretical and experimental studies have reported various novel structures of the confined water under high pressure. However, the role of this high pressure on the dynamic properties of water has not been elucidated to date. In the present study, the structure evolution and interfacial friction behavior of water constrained in a graphene nanochannel were investigated via molecular dynamics simulations. Transitions of the confined water to different ice phases at room temperature were observed in the presence of lateral pressure at the gigapascal level. The friction coefficient at the water/graphene interface was found to be dependent on the lateral pressure and nanochannel height. Further theoretical analyses indicate that the pressure dependence of friction is related to the pressure-induced change in the structure of water and the confinement dependence results from the variation in the water/graphene interaction energy barrier. These findings provide a basic understanding of the dynamics of the nanoconfined water, which is crucial in both fundamental and applied science. 

14.Self-magnetism induced large magnetoresistance at room temperature

region in graphene nanocrystallited carbon film

CARBON 112（2017）, 162–168

Chao Wang and Dongfeng Diao*

We report large positive magnetoresistance (MR) of over 12% at 273 K in graphene nanocrystallited pure carbon

 film. MR behaviors at different temperatures implied that low temperature MR was from carrier diffusive scattering and room temperature MR was from spin arrangement effect. Temperature dependences of the film resistance and magnetization recognized that as temperature decreased from 300 to 200 K, transitions occurred on the electrical transporting process from conductive mode to semi-conductive mode, and the nanocrystallited structure showed competition of ferromagnetic and antiferromagnetic interactions. The large room temperature MR was ascribed to the ferromagnetic order of spin magnetic moment arrangement at the of graphene layer edges.

=2016 = 

 13. Hydrogen-bond potential for ice VIII-X phase transition

Scientific Reports 6(2016), 37161

Xi Zhang, Shun Chen & Jichen Li

Repulsive force between the O-H bonding electrons and the O:H nonbonding pair within hydrogen bond (O-H:O) is an often overlooked interaction which dictates the extraordinary recoverability and sensitivity of water and ice. Here, we present a potential model for this hidden force opposing ice compression of ice VIII-X phase transition based on the density functional theory (DFT) and neutron scattering observations. We consider the H-O bond covalent force, the O:H nonbond dispersion force, and the hidden force to approach equilibrium under compression. Due to the charge polarization within the O:H-O bond, the curvatures of the H-O bond and the O:H nonbond potentials show opposite sign before transition, resulting in the asymmetric relaxation of H-O and O:H (O:H contraction and H-O elongation) and the H+ proton centralization towards phase X. When cross the VIII-X phase boundary, both H-O and O:H contract slightly. The potential model reproduces the VIII-X phase transition as observed in experiment. Development of the potential model may provide a choice for further calculations of water anomalies.

12. Nanobubble Skin Supersolidity

Langmuir 32(2016) , 11321−11327

Xi Zhang, Xinjuan Liu, Yuan Zhong, Zhaofeng Zhou, Yongli Huang, and Chang Q. Sun*

Water nanobubbles manifest fascinatingly higher mechanical strength, higher thermal stability, and longer lifetime than macroscopic bubbles; thus, they provide an important impact in applications in the biomedical and chemical industries. However, a detailed understanding of the mechanism behind these mysteries of nanobubbles remains a challenge. Consistency between quantum computations and Raman spectrometric measurements confirmed our predictions that a nanobubble skin shares the same supersolidity with molecular clusters, skins of bulk water, and water droplets because of molecular undercoordination (fewer than four nearest molecular neighbors). Molecular undercoordination (coordination number Zcluster < Zsurface < Zbubble < Zbulk = 4) shortens/extends the H−O/O:H bond and stiffens/softens its corresponding stretching phonons, whose frequency shift is proportional to the square root of the cohesive energy and inversely proportional to the segmental length. The strongly polarized O:H−O bond slows the molecular dynamics and increases the viscosity. The freezing temperature is lowered by the softened O:H bond, and the melting temperature is enhanced by the stiffened H−O bond. Therefore, the supersolid skin makes the nanobubbles thermally more stable, less dense, and stiffer and slows the dynamics of their molecular motion.

11.Cross-Linking-Induced Frictional Behavior of Multilayer Graphene: Origin of Friction

Tribology Letters 62 (2016), 33

Lei Yang, Qi Zhang, Dongfeng Diao*

The tribological properties of graphene have attracted intensive attentions over the past few years. It was found that the frictional behavior of multilayer graphene was dependent on the multilayer thickness. However, the origin for such phenomenon is still under discussion. In this study, the mechanism of the thickness-dependent friction was explored based on molecular dynamics simulations of the scratching process of multilayer graphene. We found that the friction coefficient dropped dramatically as the number of layers increased under the same scratch depth. Further analysis of the graphene structure variation during the scratching process showed that the amount of the crosslinking decreased when the number of layers increased, which accounts for the dependence of the friction coefficient on the thickness. Finally, a novel chanism was proposed that the thickness-dependent friction of multilayer graphene was caused by the formation of crosslinking between graphene layers. This study provides basic understanding of the origin of friction in multilayer graphene.

=2015 = 

10.From ice superlubricity to quantum friction: Electronic repulsivity and phononic elasticity

Friction 3(2015)，294-319

Xi Zhang*, Yongli Huang, Zengsheng Ma, Lengyuan Niu, Chang Qing Sun*

Superlubricity means non-sticky and frictionless when two bodies are set contacting motion. Although this occurrence has been extensively investigated since 1859 when Faraday firstly proposed a quasiliquid skin on ice, the mechanism behind the superlubricity remains uncertain. This report features a consistent understanding of the superlubricity pertaining to the slipperiness of ice, self-lubrication of dry solids, and aqueous lubricancy from the perspective of skin bond-electron-phonon adaptive relaxation. The presence of nonbonding electron polarization, atomic or molecular undercoordination, and solute ionic electrification of the hydrogen bond as an addition, ensures the superlubricity. Nonbond vibration creates soft phonons of high magnitude and low frequency with extraordinary adaptivity and recoverability of deformation. Molecular undercoordination shortens the covalent bond with local charge densification, which in turn polarizes the nonbonding electrons making them localized dipoles. The locally pinned dipoles provide force opposing contact, mimicking magnetic levitation and hovercraft. O:H−O bond electrification by aqueous ions has the same effect of molecular undercoordination but it is throughout the entire body of the lubricant. Such a Coulomb repulsivity due to the negatively charged skins and elastic adaptivity due to soft nonbonding phonons of one of the contacting objects not only lowers the effective contacting force but also prevents charge from being transited between the counterparts of the contact. Consistency between theory predictions and observations evidences the validity of the proposal of interface elastic Coulomb repulsion that serves as the rule for the superlubricity of ice, wet and dry frictions, which also reconciles the superhydrophobicity, superlubricity, and supersolidity at contacts.

 9. Tensile strain-induced magnetism transition in multilayer graphene with excess electrons: Stability of the edge-quantum well

AIP ADVANCES 5(2015), 127106

Lei Yang and Dongfeng Diao

The stability of edge-quantum well-induced strong magnetism of multilayer armchair graphene nanoribbon (AGNR) with excesselectrons was investigated under applied tensile strain by density functional theory (DFT) calculations. The results indicated that: (1) The strain along the armchair edge direction led to a transition of the multilayer AGNRs from ferromagnetic state to nonmagnetic state when the strain increased to a critical value; (2) The strain induced bond length changes reduced the stability of the edge-quantum well in terms of the reduction of the electrons capturing capacity; and (3) The spin splitting of the energy bands near the Fermi level reduced with the increase of the strain, resulting in the decrease of the spin moment. This finding suggests that the magnetic properties of graphene have strong dependence on its strain states, which is crucial to the design of graphene-based magnetic devices.

 8. Coordination-Resolved Electron Spectrometrics.

Chemical reviews 115(2015), 6746-6810

Xinjuan Liu#, Xi Zhang#, Maolin Bo#, Lei Li#,  Hongwei Tian#, Yanguang Nie#, Yi Sun#, ShiqingXu*, Yan Wang*, Weitao Zheng*, Chang Q.Sun*.

This treatise reports recent progress in resolving the atomistic, coordination-resolved, dynamic, local, and quantitative information on bond relaxation in length and energy, charge quantum entrapment and polarization, energy density, and atomic cohesive energy pertaining to the under- and heterocoordinated atoms and their joint effect. Atomic undercoordination refers to atoms associated with grain boundaries, homogeneous adatoms, point defects, solid or liquid skins, terrace edges, and nanostructures of various shapes and dimensionalities. Atomic heterocoordination means those associated with alloys, compounds, chemisorbed skins, dopants, impurities, and interfaces.

7.Potential Paths for the Hydrogen-Bond Relaxing with (H2O)(N) Cluster Size

Journal of Physical Chemistry C  119(2015), 16962-16971  

Yongli Huang#, Xi Zhang#,  Zengsheng Ma, Guanghui Zhou, Yinyan Gong, Chang Q.Sun*.

Relaxation of the hydrogen bond (O:H−O) between oxygen ions of undercoordinated molecules fascinates the behavior of water nanodroplets and nanobubbles. However, probing such potentials remains yet far from reality. Here we show that the Lagrangian solution (Huang et al. J. Phys. Chem. B 2013, 117, 13639) transforms the observed H−O bond (x = H) and O:H nonbond (x = L) lengths and their characteristic phonon frequencies (dx, ωx) (Sun et al. J. Phys. Chem. Lett. 2013, 4, 2565) into their respective force constants and cohesive energies (kx, Ex), which results in mapping of the potential paths for the O:H−O bond relaxing with (H2O)N cluster size. Results show that molecular undercoordination not only reduces its size (H−O length dH) with enhanced H−O energy from the bulk value of 3.97 to 5.10 eV for a H2O monomer but also enlarges their separation (O:H distance dL) with O:H energy reduction from 95 to 35 meV for a dimer. The H−O energy gain raises the melting point of water skin from the bulk value 273 to 310 K, and the O:H energy loss lowers the freezing temperature of a 1.4 nm sized droplet from the bulk value 258 to 202 K, which indicates droplet size induced dispersion of the quasisolid phase boundaries.

 6. A nanoscale temperature-dependent heterogeneous nucleation theory

Journal of Applied Physics 117(2015), 224303 

Y. Y. Cao and G. W. Yang*

Classical nucleation theory relies on the hypothetical equilibrium of the whole nucleation system, and neglects the thermalfluctuations of the surface; this is because the high entropic gains of the (thermodynamically extensive) surface would lead to multiple stable states. In fact, at the nanometer scale, the entropic gains of the surface are high enough to destroy the stability of the thermal equilibrium during nucleation, comparing with the whole system. We developed a temperature-dependent nucleation theory to elucidate the heterogeneous nucleation process, by considering the thermal fluctuations based on classical nucleation theory. It was found that the temperature not only affected the phase transformation, but also influenced the surface energy of the nuclei. With changes in the Gibbs free energy barrier, nucleation behaviors, such as the nucleation rate and the critical radius of the nuclei, showed temperature-dependent characteristics that were different from those predicted by classical nucleation theory. The temperature-dependent surface energy density of a nucleus was deduced based on our theoretical model. The agreement between the theoretical and experimental results suggested that the developed nucleation theory has the potential to contribute to the understanding and design of heterogeneous nucleation at the nanoscale.

 5. Nanoscratching of multi-layer graphene by molecular dynamics simulations

TribologyInternational 88 (2015) 85–88 

Qi Zhang, DongfengDiao*, MomojiKubo

Graphene or graphene-based materials are becoming a glowing material to be expected to control a frictional behavior at contact interface. However, due to its atomic layer to layer structure, it is difficult to clarify its frictional mechanism throughexperimental observation. Therefore, in this paper the frictional behavior of diamond tip nanoscratching on multi-layer graphene was investigated by Molecular Dynamics (MD) simulation. Results show super low frictional behavior of graphene layers when the scratch depth is less than 5.3 Å, when the scratch depth is over this value, the friction coefficient increases at least 10 times which is caused by phase transition of graphene layers. Besides, we discussed the sensitivity of friction coefficient to the shapes of scratch tip and its anisotropy.

 4.Hydrogen-bond relaxation dynamics: Resolving mysteries of water ice

 Coordination Chemistry Reviews, 285 (2015) 109-165  

Yongli Huang#, Xi Zhang#, Zengsheng Ma, Yichun Zhou, Weitao Zheng, Ji Zhou, Chang Q. Sun

We present recent progress in understanding the anomalous behavior of water ice under mechanical com-pression, thermal excitation, and molecular undercoordination (with fewer than four nearest neighborsin the bulk) from the perspective of

hydrogen (O:H O) bond cooperative relaxation. We modestly claimthe resolution of upwards of ten best known puzzles. Extending the Ice Rule suggests a tetrahedral blockthat contains two H2O molecules and four O:H O bonds. This block unifies the density-geometry-size-separation of molecules packing in water ice. This extension also clarifies the flexible and polarizableO:H O bond that performs like a pair of asymmetric, coupled, H-bridged oscillators with short-rangeinteractions and memory as well as extreme recoverability. Coulomb repulsion between electron pairson adjacent oxygen atoms and the disparity between the O:H and the H O segmental interactions relaxthe O:H O bond length and energy cooperatively under stimulation. A Lagrangian solution has enabledmapping of the potential paths for the O:H O bond at relaxation. The H O bond relaxation shifts themelting point, O 1s binding energy, and high-frequency phonon frequency whereas the O:H relaxationdominates polarization, viscoelasticity, and the O:H dissociation energy. The developed strategies haveenabled clarification of origins of the following observations: (i) pressure-induced proton centralization,phase transition-temperature depression and ice regelation; (ii) thermally induced four-region oscillationof the mass density and the phonon frequency over the full temperature range; and (iii) molecular-undercoordination-induced supersolidity that is elastic, hydrophobic, thermally stable, with ultra-lowdensity. The supersolid skin is responsible for the slipperiness of ice, the hydrophobicity and toughness ofwater skin, and the bi-phase structure of nanodroplets and nanobubbles. Molecular undercoordinationmediates the O:H and H O bond Debye temperatures and disperses the quasi-solid phase boundary,resulting in freezing point depression and melting point elevation. O:H O bond memory and water-skinsupersolidity ensures a solution to the Mpemba paradox — hot water freezes faster than its cold. Theseunderstandings will pave the way toward unveiling anomalous behavior of H2O

=2014 = 

3. Magnetism induced by excess electrons trapped at diamagnetic edge-quantum well in multi-layer grapheme

Applied Physics Letters, 105, 042402 (2014) 

Xi Zhang, Chao Wang, Chang Q Sun, and Dongfeng Diao*

In this paper, we clarified a robust mechanism of magnetism generated by excess electrons captured by edge-quantum well of

diamagnetic armchair edges. Consistency between density functional theory calculations and electron cyclotron resonance experiments verified that: (1) Multi-layer armchair nanoribbons are stable with proper amounts of excess electrons which can provide net spin; (2) Since under-coordination induces lattice relaxation and potential well modulation, electrons tend to be trapped at edges; and (3) Neither large amount of excess electrons nor positive charges can induce magnetism. This work shed light on the development of grapheme devices in its magnetic applications.

=2013 = 

 2. Dangling bond induced cross-linking model in nanoscratched graphene layers

 Surface &Coatings Technology 237，230（2013）

 Qi Zhang,  Dongfeng Diao*, and Lei Yang.

Dangling bond induced cross-linking in an interlayer graphene during nanoscratch is simulated bymolecular dynamics method.The normal stress over 74 GPa leads to a broken hexagonal ring of the intralayer graphene, producing unstable dangling bonds which easily make up sp2 or sp3 between neighbor layers. The cross-linking density increaseswith increasing scratching depth, causing higher scratch hardness. The maximumscratch hardness is 90 GPa. The cross-linking is reversible after scratch when the normal stress is less than 90 GPa, beyond which the atoms from different graphene layers will be mixed together forming amorphous structure, making the scratch hardness decrease sharply. These results provide insights into the structural andmechanical properties of graphene based materials.

 1. Potential of grapheme layer controlling nano-wear during C60 intrusion by molecular dynamics simulation

Wear306, 248–253 (2013)

Qi Zhang and Dongfeng Diao*

The intrusion process of a C60 (the smallest ball in nature) into a sliding contact space was simulated using the Molecular Dynamics approach. The contact space consisted of upper and lower Si substrates layered by graphene with an included angle changing from 20o to 90o, andC60 just contacted with both the upper and lower substrates. A constant horizontal speed 3nm/ps was applied to the lower substrate, and the process of C60 breaking through the contact space was observed. During this process, the nano-wear of the substrate was evaluated by the number of dropped atoms from the substrates. The results showed the number of dropped atoms from the upper Si substrate was dependent on the initial included angle and the graphene layered positions. The potential that graphene can control nano-wear and realize no-wear of the surface was revealed, also the potential that graphene can control C60 intrusion into the sliding contact space was found.