Argonne National Laboratory

2014 Colloquium Archive

Date Title
Nov. 19, 2014

"Metal-organic Framework for Sustainable Catalysis and Cancer Therapy", Wenbin Llin, University of Chicago, hosted by Yugang Sun

Abstract: Metal-organic frameworks (MOFs) represent an interesting class of crystalline molecular materials that are synthesized by combining metal-connecting points and bridging ligands. The modular nature of and mild conditions for MOF synthesis have permitted the rational structural design of numerous MOFs and the incorporation of various functionalities via constituent building blocks. The structure-property relationships of MOFs can also be readily established by taking advantage of the knowledge of their detailed atomic structures, which enables fine-tuning of their functionalities for desired applications. Through the combination of molecular synthesis and crystal engineering, MOFs present an unprecedented opportunity for the rational and precise design of functional materials.

In this talk, I will discuss our recent works on designing MOFs for sustainable catalysis and cancer therapy. MOFs have enabled the rational synthesis of single-site solid catalysts by not only facilitating the immobilization of known homogeneous catalysts but also allowing the discovery of new molecular catalysts that do not have homogeneous counterparts. Furthermore, we have demonstrated the ability to combine multiple treatment modalities into a single MOF nanoparticle for effective cancer therapy in mouse models.

Nov. 5, 2014

"Molecular Scale Understanding and Design of Solid Oxide Fuel Cathodes," Dane Morgan, University of Wisconsin-Madison, hosted by Maria Chan

Abstract: From peak oil to global warming to grid stability, a host of challenges suggest that our methods for obtaining energy will have to change dramatically over the next few decades. Solid oxide fuel cells (SOFCs) extract energy from fuels electrochemically, offering a clean, low-emission, quiet, reliable, fuel adaptable, and highly efficient way to obtain power. Many researchers are therefore exploring SOFCs for applications ranging from integration into coal and gas power plants to distributed power for buildings.

An outstanding challenge in the design of SOFCs is that they must catalyze the oxygen reduction reaction, O2(gas) +2e- → 2O2- (solid). This reaction requires good catalysts and can be done efficiently only at high temperature, limiting the durability and applicability of the fuel cells. There is therefore a strong interest in developing improved catalysts for SOFCs. Present SOFC catalysts are typically perovskite oxides, which can reduce O2 gas and transport O2- through their bulk to the electrolyte. However, this unusual chemical reaction is still poorly understood, inhibiting the development of improved SOFC technologies.

In this talk I will discuss how ab initio quantum mechanical methods can be used to better understand and design improved cathode materials. I will demonstrate the power of these approaches to predict bulk defect chemistry and surface segregation, and I will provide both a descriptor and a full molecular model of the surface catalytic processes. These studies suggest that our understanding in this class of materials is progressing to the point where full molecular prediction of the catalytic rates on general systems may soon be possible.

October 22, 2014

"Synthesis, Characterization, and Application of Tunable Resistance Coatings Prepared by Atomic Layer Deposition," Jeffrey Elam, Energy Systems Division, Argonne National Laboratory, hosted by Seth Darling

Recently we have used atomic layer deposition (ALD) to synthesize nanocomposite coatings comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These films are comprised of M:Al2O3 where M = W or Mo, and are prepared using alternating exposures to trimethyl aluminum (TMA) and H2O for the Al2O3 ALD and alternating MF6/Si2H6 exposures for the metal ALD. By varying the ratio of ALD cycles for the metal and the Al2O3 components in the film, we can tune precisely the resistance of these coatings over a very broad range from 1012 to 105 Ohm-cm. These films exhibit Ohmic behavior and resist breakdown even at high electric fields of 107 V/m. Moreover, the self-limiting nature of ALD allows us to grow these films inside of porous substrates and on complex three-dimensional surfaces. As a result of these qualities, these nanocomposite films have proved to be exceptional as charge drain coatings in electro-optical MEMS devices, and as resistive coatings in solid-state electron multipliers (microchannel plates [MCPs]).

To investigate the growth mechanism for the ALD composite films we employed in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) absorption spectroscopy studies. In the case of the Mo:Al2O3 films, QCM showed that the Mo ALD inhibits the Al2O3 ALD and vice versa. Despite this inhibition, the relationship between Mo content and Mo cycle percentage was close to expectations. Surprisingly, FTIR revealed that the reducing agent for the Mo is not the Si2H6, but rather the TMA exposure from the subsequent Al2O3 ALD cycle. Depth profiling X-ray photoelectron spectroscopy showed that the M:Al2O3 films are uniform in composition and contained Al, O, and metallic Mo or W as expected, but also include significant F and C. Cross-sectional transmission electron microscopy revealed the film structure to be metallic nanoparticles (~1 nm) embedded in an amorphous matrix.

We have used these nanocomposite coatings to functionalize capillary glass array plates to fabricate large-area MCPs suitable for application in large-area photodetectors. In addition, we have applied these films to serve as charge drain coatings in MEMS devices for a prototype electron-beam lithography tool, and obtained high-resolution electron-beam patterns without charging artifacts.

October 8. 2014

"Two-Dimensional Electronic Materials and Phenomena: Graphene and Beyond," Randall Feenstra, Carnegie Mellon University, hosted by Saw Wai Hla

The past decade has witnessed a great surge of activity in two-dimensional materials, particularly focused on graphene. The novel electronic band structure of this semimetal, in the form of Dirac cones, leads to a range of interesting physical phenomena and applications. More recently, other two-dimensional materials such as hexagonal boron nitride (an insulator) or MoS2 and related materials (typically semiconductors) have been combined with graphene to form two-dimensional vertical heterostructures, with new electronic properties. In this talk, a few examples of recent studies of such materials will be presented, emphasizing the novel electronic states and how these can be measured (as a means of characterizing the materials) as well as employed for applications. A range of materials systems will be discussed, including two-dimensional electrons gases in complex oxide surfaces such as SrTiO3(100).

Special Colloquium

October 7, 2014
4:00-5:00 p.m.
Bldg. 440, A105-106

"Mesoscopic Oscillators: A Glimpse into Quantum Fluctuations Far from Equilibrium," Mark Dykman, Michigan State University, hosted by Daniel Lopez

Mesoscopic vibrational systems are studied in many areas, from nanomechanics to cavity quantum electrodynamics to Josephson junctions. Besides various applications, they allow one to address a general problem of quantum fluctuations in systems away from thermal equilibrium. We will show that these fluctuations display unusual features, including the mechanism of switching between coexisting stable periodic states that has no analog in equilibrium systems. We call it quantum activation. The scaling behavior of the switching rates will be outlined and a comparison with the experiment will be made. The effect of fragility of the rates of rare events in nonequilibrium systems will be also discussed.

Sept. 24, 2014

"Nanoplasmonics: Reaching Out to the Single Molecule," Reuven Gordon, University of Victoria, hosted by Jeff Guest

Abstract: Nanoplasmonics refers to the nanostructuring of metals to achieve enhanced light-matter interaction, down to the single-molecule level. This talk will highlight some of our research advances toward single-molecule (and other) technologies.

In particular, I will discuss reliable single-molecule Raman by using the antenna concept of directivity. I will show how we can grab hold of single proteins with light, unfold them, and detect single-protein binding with great sensitivity. Nanohole array surface plasmon resonance sensing with 10-7 refractive index unit resolution will be discussed. I will show how nanoplasmonics can improve THz sources and detectors, and can allow for enhanced second-harmonic generation. I will also discuss the quantum limits of plasmonics and show recent experimental evidence of the onset of these limits using third-harmonic generation. Finally, I will talk about large field enhancements that can be achieved by combining nanoplasmonics with permittivity-near-zero materials.

Sept. 10, 2014

"Inorganic Control of Biological Self-Assembly," Akif Tezcan, University of California, San Diego, hosted by Chris Fry

One-, two- and three-dimensional protein arrays play central roles in diverse biological processes, are widely used in nano- and bio-technological applications, and they form the basis of protein crystallography. The design of an arbitrary protein that can self-assemble into such arrays is very desirable, but it is complicated by the chemical heterogeneity of protein surfaces. We have shown that through enginereed metal coordination, monomeric proteins can be assembled into helical one-dimensional nanotubes and two- and three-dimensional arrays with crystalline order. These assemblies show striking similarities to highly evolved architectures such as microtubules and bacterial S-layers in terms of their shapes, dimensions and structural uniformity, as well as their responsiveness to chemical triggers. The assemblies can be uniformly functionalized on the level of an individual building block, providing a route for creating homogeneous nanoscale materials.

August 27, 2014

"Multiparadigm Computational Approaches In Analysis and Design of Energy Harvesting and Storage Materials," Tahir Cagin, Texas A&M University, hosted by Alper Kinaci

Abstract: Based on various levels of theory, we use different computational paradigms to analyze and assess the efficiency and utility of different materials and materials systems in both three-dimensional bulk and lower-dimensional nanostructures. We employ initio quantum chemistry, density functional theory (DFT), molecular mechanics, molecular dynamics, and molecular dynamics simulations, as well as engineering level methods to determine relevant properties (both static and dynamic) and relevant coupling coefficients for energy conversion to assess the figure of merit. In this talk, we present this multiparadigm approach as it is applied to thermoelectrics, piezoelectrics, and H-storage materials.

For thermoelectrics, we determine properties such as: Seebeck coefficient, electronic conductivity, and thermal conductivity of materials to assess their feasibility in cooling and power generation applications. The efficiency for both applications of thermoelectric materials is slowly increasing function of the figure of merit, which is a function of these particular transport properties. We will present the underlying theory and computational approaches used in determining these properties and discuss applications for bulk and low-dimensional nanostructured materials. Examples include Bi2Te3, Sb2Te3, and their superlattices; pure-SrTiO3, doped-SrTiO3 and SrTiO3-based perovskite alloys; various ternary and quaternary alloys; and one- and two-dimensional nanostructures such as carbon and BN nanotubes, and graphene and nano-ribbons.

For piezo-electrics, using ab initio DFT and polarizable-charge transfer interaction potentials in molecular dynamics simulations, we determine the piezoelectric coefficients as well as variation of polarization as a function of chemical constitution and nanostructure in ABO3 ceramics.

For H-Storage applications we use molecular dynamics and Grand Canonical Monte Carlo simulations to assess storage capacity of MOFs and CNT-based scaffolds.

July 23, 2014

"Designing Complexity in Silicon Nanostructures for Enhanced Bio-interfaces," Bozhi Tian, University of Chicago, hosted by Leo Ocola

Abstract: Biological systems are rich with electrical and mechanical activities. Alongside the well-known pathways of biochemical regulation, there exist additional pathways of biological communication governed not by chemical reagents, but by electrical and mechanical signals. Silicon-based nanoscale materials and devices represent diverse and powerful tools for achieving nanoscale bioelectric interfaces with cells and tissue. For example, silicon nanowire field effect transistors exhibit exquisite sensitivity in chemical and biological detection and can form strongly coupled electrical interfaces with cellular components. My talk will focus on several biomimetic design considerations toward breaking down the boundary between nonliving and living systems across multiple length scales. I will describe how we experimentally apply these designs in the nanoelectronic systems for building minimally invasive interfaces with single cells and subcellular components. Specifically, I will discuss a new synthetic approach to realize the biomineral-like probes for extracellular measurements. Finally, I will describe the prospects in future fundamental studies and applications in the life sciences.

July 16, 2014

"In Vivo Brain Optical Imaging as a Tool for Studying Animal Models of Neural Disease," Vassilliy Tsytsarev, University of Maryland, hosted by Elena Rozhkova

Abstract: Brain optical imaging is based on the idea that the optical features of neurons are highly responsive to functional changes in the neural tissue, and light of different wavelengths can be used to observe the morphological structure and function in vivo in the exposed brain as well as transcranially. In this presentation, we review the current optical approaches used for in vivo imaging of neural activity as well as neurovascular coupling events in small animal models. The basic principles of each technique — optical imaging of intrinsic signal (IOS), voltage-sensitive dye imaging (VSDi), and photoacoustic tomography (PA) — are described in detail, followed by examples of current applications from cutting-edge studies of cerebral neurovascular coupling functions and metabolic functions.

Using VSDi, we visualized neural activity in the rat somatosensory cortex in response to the deflection of a single whisker in different directions. The data obtained indicates that fast movement of single whiskers in varying directions correlate with different patterns of activation in the neocortex, and a functional map was created.

Also we will provide a glimpse into the ways in which these techniques might be translated to human studies for clinical investigations of pathophysiology and disease. For example, epilepsy mapping with high spatial and temporal resolution has a great significance for both fundamental research of epileptic seizures and the clinical management of this disease. In our study, we used PA to monitor hemodynamic changes in the microvasculature surrounding the epileptic neurons. A large vasodilatation of the blood vessels in the area of the epileptic foci signals the onset of the seizure. Although it is unlikely that vasodilatation itself can initiate the seizure, fast vasodilatation might be a first indicator of the local biochemical process that accompanies initiation of the seizure. In vivo optical imaging techniques continue to expand and evolve, allowing us to discover the fundamental basis of neurovascular coupling roles in cerebral physiology.

July 2, 2014

"Plasmonics for Subwavelength Light Control in the Quantum And Classical Regime," Yongmin Liu, Northeastern University, hosted by Jun Suk Rho

Abstract: Plasmonics has become a very important branch in nano optics, focusing on the new physical phenomena and unique applications of surface plasmons occurring in metallic nanostructures. Plasmonics allows us to concentrate, guide, and manipulate light at the deep subwavelength scale, promising enhanced light-matter interaction, next-generation optical circuits, sub-diffraction-limited imaging, efficient energy harvesting, and ultrasensitive biomedical detection. Furthermore, the assembly of metallic nanostructures can be used to construct optical metamaterials with exotic properties and functionalities, including artificial magnetism, negative refraction, and invisibility cloak. I will present some of our work in the fascinating field of plasmonics.

First, we demonstrate that plasmonic nanostructures can significantly modify the photonic density of states, and enhance the spontaneous emission. Such a strong Purcell effect can suppress photo bleaching, a photochemical reaction that permanently damages fluorescent molecules. As a result, a single molecule can emit up to 1,000 times more photons before bleaching. Second, I will present a fully subwavelength and efficient nano-plasmonic source for unidirectional generation of surface plasmons, which is a key building block for the next generation of ultra-fast and ultra-compact integrated optical circuits. By tailoring the relative phase at resonance and the separation between two magnetic metamaterial resonators, surface plasmons can be steered to predominantly propagate along one specific direction. Finally, I will introduce a new concept of transformation plasmonics to mold near-field plasmon waves at the metal-dielectric interface in a prescribed manner. For instance, this approach enables surface plasmon waves to travel smoothly at uneven surfaces, where surface plasmons would normally suffer considerable scattering losses. Some plasmonic devices, such as a plasmonic Luneburg lens and reconfigurable plasmofluidic lenses, will also be presented.

June 4, 2014

"Making Nanophotonics Devices a Reality: Nanofabrication of Advanced Nanophontic Structures," Stephano Cabrini, Lawrence Berkeley National Laboratory Molecular Foundry, hosted by Elena Shevchenko

Abstract: To exploit the potential of nanophotonics, it is important to control material properties at the nanometer scale, obtaining good agreement between experiments and theory. Nanofabrication can open the way to new concepts of devices. In this presentation, we will present some of the devices fabricated and tested at the Molecular Foundry. We will show new plasmonic pyramidal antennas able to perform efficient adiabatical optical compression and suitable to be used as near-field scanning optical microscopy probes; a perfect plasmonic coaxial resonator made by helium ion beam lithography; and a photonic planar hologram used as an integrated spectrometer.

May 21, 2014

"The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells and Charge Dynamics," by Harald Ade, North Carolina State University, hosted by Seth Darling

Abstract: In bulk heterojunction (BHJ) organic photovoltaics (OPVs), electron donating and electron accepting materials form a distributed network of heterointerfaces in the photoactive layer where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces because of their complex three-dimensional arrangement and the lack of techniques to measure local order. I will present results that show that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, BHJ solar cells. Using resonant soft X-ray scattering, my group characterizes the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on orientation (-1) relative to the discrete donor/acceptor heterointerfaces. By manipulating the degree of molecular orientation through choice of molecular chemistry and processing solvent characteristics, we are able to show the importance of this structural parameter on the performance of BHJ OPV devices. A complete description and theoretical modeling yet to be developed for OPVSs will have to take such molecular orientation distributions into account.

April 23, 2014

"Designing Materials at the Nanoscale: Semiconductor and Graphene Quantum Dots," by Pawel Hawrylak, National Research Council of Canada, hosted by Stephen Gray

Abstract: We review here recent theoretical and experimental results on designing materials at the nanoscale: semiconductor and graphene quantum dots. We briefly describe lateral quantum dot molecules as building blocks of quantum circuits based on electron spin: artificial Haldane gap device, GHZ maximally entangled state, Berry's phase, and e-e driven topological phase generators. We next turn to the locking of spin and orbital motion and emergence of tunable topologically protected chiral surface states in HgTe quantum dots. Finally, we describe one-atom-thick semiconductor quantum dots made of graphene and compare them with semiconductor quantum dots. We show how their electronic, optical, and magnetic properties can be engineered by the size, shape, type of edge, topology, and number of layers. Preliminary comparison of theory with experiment on optical properties of colloidal graphene quantum dots will be made.

March 26, 2014

"Nanomaterials and Human Health: The Up and the Downside as Seen from the Perspective of the Nano-Bio Interface," by Andre Nel, University of California, Los Angeles, hosted by Elena Rozhkova and Tijana Rajh

Abstract: We have come to recognize that much of biology is executed at the nanoscale level, therefore providing a rational approach to using discovery about the structure and function of engineered nanomaterials (ENMs) at the nano-bio interface for interrogation of disease, diagnosis, treatment, and imaging at levels of sophistication not possible before. Moreover, the behavior of ENMs at the nano-bio interface also constitutes the basis for hazard generation and is therefore key for understanding the safety assessment and safer design of nanomaterials.

I will discuss how discovery at the molecular, cellular, organ, and systemic nano-bio interfaces has helped us to make progress in nanomedicine and nanotoxicology. I will explain how the physicochemical properties of nanomaterials relate to nanoscale interactions at the membrane, intracellular organelles, tissues, and organs in response to exposure to a variety of commercial ENMs as well as for therapeutic nanocarriers. I will delineate how the use of high-throughput screening to establish structure-activity relationships can be used for the design of improved nanocarriers for cancer treatment as well as hazard and risk ranking of large categories of commercial ENMs on their way to the marketplace.

March 12, 2014

Ag-Au Bimetallic Nanocubes with Enhanced SERS Property and Chemical Stability," by Dong Qin, Georgia Institute of Technology, hosted by Yugang Sun

Abstract: Surface-enhanced Raman scattering (SERS) relies on the localized surface plasmon resonance and enhancement of electromagnetic fields around metal nanostructures to drastically increase the Raman scattering cross sections of molecules in close proximity to the nanostructures. It has been documented that silver nanocubes embrace SERS properties with enhancement factors up to 106 at visible excitation wavelengths for highly sensitive detection of chemical or biological species. Unfortunately, elemental silver is highly susceptible to oxidation under conditions that involve oxidants, halide ions, acids, water, ultraviolet irradiation, and heating. Such chemical instability often results in changes to the morphology of silver nanostructures, particularly at corners and edges with high surface free energies, and ultimately compromise their performance in SERS. One potential solution to improve the chemical stability of silver nanostructures is to form alloy with a more stable metal such as gold. In this talk, I will report an approach to complementing the galvanic replacement reaction between silver nanocubes and HAuCl4 with co-reduction by a reducing agent for the formation of Ag-Au hollow nanostructures with enrichment of silver to greatly enhance SERS activity. Additionally, I will report our latest development in the replacement-free seeded growth of gold on silver nanocubes with excellent SERS property and chemical stability.

February 26, 2014

Rationally Ddesigned Iron Oxide Nanoparticles as Efficient MRI Contrasts Agents," by Yuping Bao, University of Alabama, hosted by Elena Rozhkova

Magnetic resonance imaging (MRI) offers a powerful, noninvasive tool for brain tumor imaging and therapy monitoring. The use of contrast agents significantly enhances the image contrasts, yielding better resolution. T1 positive contrast agents are mainly paramagnetic gadolinium (Gd) complexes, which shorten the longitudinal relaxation time (T1) and generate a brighter image. T2 negative contrast agents, primarily superparamagnetic iron oxide nanoparticles, produce a darker image by shortening the transverse relaxation time (T2). Compared with the well-studied Gd-based contrast agents, the correlation between the nanoparticle parameters and corresponding relaxivities are not well understood. In this presentation, we will use a suite of magnetic nanoparticles with well-designed parameters to address the correlation between the nanoparticle parameters and relaxivities. The hydrophilic coating thickness directly impacts the R2 relaxivity, but shown minimal effects on R1 relaxivity. In particular, the nanoparticle shape plays an important role as MRI contract agents. Along with the exploration of the potential of iron oxide nanoparticles as MRI contrast agents, the reproductive toxicity of these nanoparticles were also investigated on drosophila fly and pregnant mice.

February 12, 2014

Using Molecular Dynamics Simulations to Advance our Understanding of Complex Biological Systems," by Benoit Roux, University of Chicago, hosted by Stephen Gray

Abstract: Molecular dynamics simulation of large biological macromolecules have reached the point where they can be used to provide meaningful insight on the function of complex systems. Free-energy methodologies are particularly important to establish a strong connection to experiments. This will be illustrated with a few recent examples on

  • Src tyrosine kinases
  • K+ channels
  • Sodium-potassium ATPase pump.
January 15, 2014

Surface Passivation and Oxidation of PbSe and PbS Quantum Dots: Theoretical Insights," by Svetlana Kilina, North Dakota State University, hosted by Richard Schaller

Abstract: Quantum dots (QDs) show promise for many technological applications, including photocatalysis and photovoltaics. However, their photophysical properties are sensitive to surface reactions, resulting in uncontrollable luminescence quenching.

Using density functional theory (DFT) and time-dependent DFT (TDDFT), we simulate the oxidation process on the surface of Pb16Se16 and Pb68S68 QDs and its effect on their electronic and optical properties. When oxygen is substituted for Se/S ions, the electronic properties of the QD are insignificantly perturbed. In contrast, if atomic oxygen is adsorbed on the QD surface and coordinated with two lead ions, it introduces additional unoccupied states inside the QD’s band gap, so-called mid-gap trap states. Such states are hybridized between the oxygen and the QD’s surface atoms and contribute to the lowest energy optically dark or semidark transitions, resulting in quenching of QD luminescence. In contrast, if the oxygen is coordinated with Se/S and lead ions on the surface, the mid-gap states are not present, and the optical transitions are similar to those of nonoxidized QDs.

We have also observed similar trends when a chlorine radical is adsorbed to the QD surface: a few trap states originating from chlorine appear at the band gap of the QD, when chlorine is coordinated with lead ions. However, when ionized, the interaction of Cl- with the QD surface leads to elimination of trap states from the band gap and a slight increase in the gap of the QD. Our calculations demonstrate different preferential binding of chlorine in its radical and ionized forms to different QD surfaces. Thus, the chlorine ion has a stronger interaction with the {111} PbSe surface, while the chlorine radical has similar binding energies to {100} and {111} surfaces. Attachment of chlorine in the form of PbCl2 salt favors adsorption to the {111} surface of the QD. However, its binding energy is twice smaller than adsorbed chlorine ions and radicals on the same surface, while it also eliminates trap states from the QD’s band gap.

The obtained results is a first step in understanding the physical properties of QDs found in the presence of defects, surface ligands, and interactions with the environment. We will further demonstrate how these properties could be controlled to accelerate the completion of their proof-of-concept development stage and facilitate practical usage of hybrid nanomaterials to produce efficiently operating optoelectronic devices, solar cells, sensors, bio-labels, etc.

January 15, 2014

Friction, Brownian Motion, and Energy Dissipation Mechanisms in Adsorbed Molecules and Molecularly Thin Films: Heating, Electrostatic and Magnetic Effects," by Jacquelin Krim, North Carolina State University, hosted by Diana Berman

Abstract: In the study of friction at the nanoscale, phononic, electrostatic, conduction electron, and magnetic effects all contribute to the dissipation mechanisms. Electrostatic and magnetic contributions are increasingly alluded to in the current literature, but they remain poorly characterized. I will first overview the nature of these various contribution, and then report on our observations of magnetic and electrostatic contributions to friction for various systems in the presence and absence of external fields. I will also report on the use of a quartz crystal microbalance with a graphene/Ni(111) electrode to probe frictional heating effects in Kr monolayers sliding on the microbalance electrode in response to its oscillatory motion.