We combine the tools of statistical mechanics with direct imaging at the atomic scale, using scan... more We combine the tools of statistical mechanics with direct imaging at the atomic scale, using scanning tunneling microscopy (STM) to observe and characterize intrinsic fluctuation properties on surfaces and interfaces, and further explore how these properties are affected by current, the electromigration problem. Fluctuation properties of steps and islands on metal surfaces, twin boundaries on metal nanowires, are investigated and the underneath mass transport mechanisms are determined. Then we has studied how mass transport properties are affected by undesirable impurity on surfaces or deposited molecules, especially we focus on island shape change and fluctuations of molecule/metal interfaces. For the organic-metal system, we further investigate related electromigration properties, and find island migration and step bend due to wind force. Finally, organic thin films are also investigated, including the novel 2D chiral structure, and phase boundary fluctuations in an organic thin f...
Science in China Series E: Technological Sciences, 2001
We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic ... more We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic graphite in ultrahigh vacuum. Studies of ultrahigh vacuum scanning tunneling microscopy revealed that the size distribution of gold clusters was very narrow and quasi-one-dimensional chains of gold nanoclusters of approximately 2 nm diameter were produced after being annealed at 74°C. Unlike the results obtained by
The dependence of supramolecular structures on fractional molecular coverage in a 2-component adl... more The dependence of supramolecular structures on fractional molecular coverage in a 2-component adlayer has been investigated using scanning tunneling microscopy. A series of acridine-9-carboxylic acid (ACA) surface structures emerges sequentially when deposited on Ag(111) at room temperature. At low molecular coverage (theta0.4ML), then a dimer structure composed of ACA dimers linked by carboxyl-carboxyl hydrogen bonds (theta˜1.0ML). The structures of the
Low-dimensional boundaries between phases and domains in organic thin films are important in char... more Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111), have been visualized in real time, and measured quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room
In this work, some most frequently studied interconnect solutions in recent years have been discu... more In this work, some most frequently studied interconnect solutions in recent years have been discussed and compared to show their advantages and limitations. RF wireless interconnect (RFWI) method is further studied by system modeling in simulink environment and successful communication between transmitter and receiver is obtained. The results indicate the potential feasibility and reliability of the inter-chip and intra-chip communication system using integrated RF transceiver and antenna to replace the present metal wiring interconnect. The fact that lots of the existing experiences on RF transceiver and antenna design makes RFWI an attractive interconnect solution.
Atomic motions on the fundamental time scale of a single atomic vibrational period (100 femtoseco... more Atomic motions on the fundamental time scale of a single atomic vibrational period (100 femtosecond to ˜1 picosecond) ultimately determine the evolution of new phases in solids, the kinetic pathways of chemical reactions, and the biological functioning processes. However, the direct observation of these structural dynamics has been limited by the current available experimental tools. In the past few years, we have developed the femtosecond electron diffraction (FED) with the capability of directly measuring the ultrafast structural dynamics in solid materials on this fundamental time scale. With the FED, a diffraction pattern with significant signal-to-noise ratio to determine the long-range order in solids can be obtained with only single femtosecond electron pulse. The design and performances of femtosecond electron gun and two-dimensional single electron detector, which are the key components of the FED, will be presented.
We report the direct observation of the ultrafast laser-induced structural change in thin metal f... more We report the direct observation of the ultrafast laser-induced structural change in thin metal films on the sub-picosecond timescale with femtosecond electron diffraction. Upon femtosecond laser excitation, a concerted motion of Bragg peaks was observed, where all the peaks start to shrink to the zero-order spot and reach maximum shift in less than one picosecond, followed by a recovery to their original positions on several tens of picoseconds. The maximum Bragg peak shift corresponds to ˜ 1% expansion of lattice spacing. The timescale of this process is found sample and pump power dependent. Possible mechanism of this shift will be discussed.
We report a direct and real-time measurement of both coherent and thermal atomic motions using fe... more We report a direct and real-time measurement of both coherent and thermal atomic motions using femtosecond electron diffraction , which provides for the first time a clear atomic-level view of laser-induced structural dynamics.
… Electronics and Applications (ICIEA), 2011 6th …, 2011
... For example, the classical Johnson-Kendall-Roberts (JKR) model [7], the Hertz model, the Derj... more ... For example, the classical Johnson-Kendall-Roberts (JKR) model [7], the Hertz model, the Derjaguin-Müller-Toporov (DMT) model [8, 9] and the Maugis-Dugdale (MD) model are respectively adopted to study adhesion mechanism of micromanipulation [10]. ...
In nanoscale metal wires, electrical current can cause structural changes through electromigratio... more In nanoscale metal wires, electrical current can cause structural changes through electromigration, in which the momentum of electrons biases atomic motion, but the microscopic details are complex. Using in situ scanning tunneling microscopy, we examined the effects of thermally excited defects on the current-biased displacement of monatomic islands of radius 2 to 50 nanometers on single-crystal Ag(111). The islands move opposite to the current direction, with velocity varying inversely with radius. The force is thus in the same direction as electron flow and acts on atomic defect sites at the island edge. The unexpectedly large force on the boundary atoms can be decreased by over a factor of 10 by adding a mildly electron-withdrawing adsorbate, C60, which also modifies the step geometry. The low coordination of the identified scattering sites is the likely origin of the large force.
The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 30... more The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 300 K, dramatically changes the nanocrystalline shape and fluctuations of the islands. We tune coverage so that a single chain of C60 fully decorates each Ag island boundary, forming a closed circular "necklace." We model the C60-induced rounding in terms of competing energetic and entropic effects. We thereby characterize the decorated-step fluctuations and estimate the C60-Ag and C60-C60 attractions to be approximately 0.13 and approximately 0.03 eV, respectively. Generalizations of our model show that decorating molecules of both circular and rectangular surface-projected symmetry will similarly lower the energy of fully kinked boundaries, leading to corner rounding and reorientations by 30 degrees on (111) surfaces and 45 degrees on (100) surfaces.
We combine the tools of statistical mechanics with direct imaging at the atomic scale, using scan... more We combine the tools of statistical mechanics with direct imaging at the atomic scale, using scanning tunneling microscopy (STM) to observe and characterize intrinsic fluctuation properties on surfaces and interfaces, and further explore how these properties are affected by current, the electromigration problem. Fluctuation properties of steps and islands on metal surfaces, twin boundaries on metal nanowires, are investigated and the underneath mass transport mechanisms are determined. Then we has studied how mass transport properties are affected by undesirable impurity on surfaces or deposited molecules, especially we focus on island shape change and fluctuations of molecule/metal interfaces. For the organic-metal system, we further investigate related electromigration properties, and find island migration and step bend due to wind force. Finally, organic thin films are also investigated, including the novel 2D chiral structure, and phase boundary fluctuations in an organic thin f...
Science in China Series E: Technological Sciences, 2001
We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic ... more We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic graphite in ultrahigh vacuum. Studies of ultrahigh vacuum scanning tunneling microscopy revealed that the size distribution of gold clusters was very narrow and quasi-one-dimensional chains of gold nanoclusters of approximately 2 nm diameter were produced after being annealed at 74°C. Unlike the results obtained by
The dependence of supramolecular structures on fractional molecular coverage in a 2-component adl... more The dependence of supramolecular structures on fractional molecular coverage in a 2-component adlayer has been investigated using scanning tunneling microscopy. A series of acridine-9-carboxylic acid (ACA) surface structures emerges sequentially when deposited on Ag(111) at room temperature. At low molecular coverage (theta0.4ML), then a dimer structure composed of ACA dimers linked by carboxyl-carboxyl hydrogen bonds (theta˜1.0ML). The structures of the
Low-dimensional boundaries between phases and domains in organic thin films are important in char... more Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111), have been visualized in real time, and measured quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room
In this work, some most frequently studied interconnect solutions in recent years have been discu... more In this work, some most frequently studied interconnect solutions in recent years have been discussed and compared to show their advantages and limitations. RF wireless interconnect (RFWI) method is further studied by system modeling in simulink environment and successful communication between transmitter and receiver is obtained. The results indicate the potential feasibility and reliability of the inter-chip and intra-chip communication system using integrated RF transceiver and antenna to replace the present metal wiring interconnect. The fact that lots of the existing experiences on RF transceiver and antenna design makes RFWI an attractive interconnect solution.
Atomic motions on the fundamental time scale of a single atomic vibrational period (100 femtoseco... more Atomic motions on the fundamental time scale of a single atomic vibrational period (100 femtosecond to ˜1 picosecond) ultimately determine the evolution of new phases in solids, the kinetic pathways of chemical reactions, and the biological functioning processes. However, the direct observation of these structural dynamics has been limited by the current available experimental tools. In the past few years, we have developed the femtosecond electron diffraction (FED) with the capability of directly measuring the ultrafast structural dynamics in solid materials on this fundamental time scale. With the FED, a diffraction pattern with significant signal-to-noise ratio to determine the long-range order in solids can be obtained with only single femtosecond electron pulse. The design and performances of femtosecond electron gun and two-dimensional single electron detector, which are the key components of the FED, will be presented.
We report the direct observation of the ultrafast laser-induced structural change in thin metal f... more We report the direct observation of the ultrafast laser-induced structural change in thin metal films on the sub-picosecond timescale with femtosecond electron diffraction. Upon femtosecond laser excitation, a concerted motion of Bragg peaks was observed, where all the peaks start to shrink to the zero-order spot and reach maximum shift in less than one picosecond, followed by a recovery to their original positions on several tens of picoseconds. The maximum Bragg peak shift corresponds to ˜ 1% expansion of lattice spacing. The timescale of this process is found sample and pump power dependent. Possible mechanism of this shift will be discussed.
We report a direct and real-time measurement of both coherent and thermal atomic motions using fe... more We report a direct and real-time measurement of both coherent and thermal atomic motions using femtosecond electron diffraction , which provides for the first time a clear atomic-level view of laser-induced structural dynamics.
… Electronics and Applications (ICIEA), 2011 6th …, 2011
... For example, the classical Johnson-Kendall-Roberts (JKR) model [7], the Hertz model, the Derj... more ... For example, the classical Johnson-Kendall-Roberts (JKR) model [7], the Hertz model, the Derjaguin-Müller-Toporov (DMT) model [8, 9] and the Maugis-Dugdale (MD) model are respectively adopted to study adhesion mechanism of micromanipulation [10]. ...
In nanoscale metal wires, electrical current can cause structural changes through electromigratio... more In nanoscale metal wires, electrical current can cause structural changes through electromigration, in which the momentum of electrons biases atomic motion, but the microscopic details are complex. Using in situ scanning tunneling microscopy, we examined the effects of thermally excited defects on the current-biased displacement of monatomic islands of radius 2 to 50 nanometers on single-crystal Ag(111). The islands move opposite to the current direction, with velocity varying inversely with radius. The force is thus in the same direction as electron flow and acts on atomic defect sites at the island edge. The unexpectedly large force on the boundary atoms can be decreased by over a factor of 10 by adding a mildly electron-withdrawing adsorbate, C60, which also modifies the step geometry. The low coordination of the identified scattering sites is the likely origin of the large force.
The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 30... more The decoration of hexagonal Ag/Ag(111) monolayer islands by chains of C60, observed via STM at 300 K, dramatically changes the nanocrystalline shape and fluctuations of the islands. We tune coverage so that a single chain of C60 fully decorates each Ag island boundary, forming a closed circular "necklace." We model the C60-induced rounding in terms of competing energetic and entropic effects. We thereby characterize the decorated-step fluctuations and estimate the C60-Ag and C60-C60 attractions to be approximately 0.13 and approximately 0.03 eV, respectively. Generalizations of our model show that decorating molecules of both circular and rectangular surface-projected symmetry will similarly lower the energy of fully kinked boundaries, leading to corner rounding and reorientations by 30 degrees on (111) surfaces and 45 degrees on (100) surfaces.
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