TEM at millikelvin temperatures: Observing and utilizing superconducting qubits.

We present a case for developing a millikelvin-temperature transmission electron microscope (TEM). We start by reviewing known reasons for such development, then present new possibilities that have been opened up by recent progress in superconducting quantum circuitry, and finally report on our ongoing experimental effort. Specifically, we first review possibilities to observe a quantum mechanically superposed electromagnetic field around a superconducting qubit. This is followed by a new idea on TEM observation of microwave photons in an unusual quantum state in a resonator. We then proceed to review potential applications of these phenomena, which include low dose electron microscopy beyond the standard quantum limit. Finally, anticipated engineering challenges, as well as the authors' current ongoing experimental effort towards building a millikelvin TEM are described. In addition, we provide a brief introduction to superconducting circuitry in the Appendix for the interested reader who is not familiar with the subject.

Frequency-Dependent Piezoresistive Effect in Top-down Fabricated Gold Nanoresistors.

Piezoresistive strain gauges allow for electronic readout of mechanical deformations with high fidelity. As piezoresistive strain gauges are aggressively being scaled down for applications in nanotechnology, it has become critical to investigate their physical attributes at different limits. Here, we describe an experimental approach for studying the piezoresistive gauge factor of a gold thin-film nanoresistor as a function of frequency. The nanoresistor is fabricated lithographically near the anchor of a nanomechanical doubly clamped beam resonator. As the resonator is driven to resonance in one of its normal modes, the nanoresistor is exposed to frequency-dependent strains of ε ≲ 10-5 in the 4-36 MHz range. We calibrate the strain using optical interferometry and measure the resistance changes using a radio frequency mix-down technique. The piezoresistive gauge factor γ of our lithographic gold nanoresistors is γ ≈ 3.6 at 4 MHz, in agreement with comparable macroscopic thin metal film resistors in previous works. However, our γ values increase monotonically with frequency and reach γ ≈ 15 at 36 MHz. We discuss possible physics that may give rise to this unexpected frequency dependence.

On the Dynamics of Intrinsic Carbon in Copper during the Annealing Phase of Chemical Vapor Deposition Growth of Graphene.

In chemical vapor deposition (CVD) growth of graphene, intrinsic carbon in copper has been shown to play a role, especially during the nucleation phase. Here, we report experimental results on depletion of carbon from the bulk of a Cu foil to its surface at different hydrogen pressures, which explain new aspects of the interplay between hydrogen and intrinsic carbon prior to growth. We observed that rising H2 pressure boosts carbon depletion to the surface, but at the same time, at elevated H2 pressures, the graphitic film formed on the Cu surface is etched away at a faster rate. This effect led us to practice annealing of copper under high hydrogen pressure as an approach to decrease the total content of carbon in the copper foil and consequently reducing the nucleation density of graphene flakes. These results enhance our understanding about the role of H2 in the CVD process and explain some of the inconsistencies among the earlier reports.

Superbat: battery-like supercapacitor utilized by graphene foam and zinc oxide (ZnO) electrodes induced by structural defects.

The current work presents a hybrid type of energy storage device composed of both graphene foam and zinc oxide electrodes, which exhibits both the electrochemical performance of a supercapacitor with a relatively higher power density, and a battery with a relatively higher energy density as compared to each individual component as single devices. Te hybrid's improved performance was correlated to the defective structure of the electrodes. To enhance the electrochemical performance of supercapacitors, it is necessary to have a well-defined mass, shape, and surface area of electrode materials. Here, we present an original design of a mounting device that enabled precisely determining all the critical parameters of electrode materials for a particular mass and surface area. With the aid of our original setup, we produced a supercapacitor device that could also act as a battery due to its high energy density values, hence we named it as superbat. In this work, 3D graphene foam was used as the first electrode due to its large surface, while for the second electrode, ZnO nanocrystals were used due its defective structure. Paramagnetic resonance Raman and impedance spectroscopy were performed in order to understand the origin of the performance of the hybrid capacitor in more depth. In particular, we obtained a high specific capacitance value (C = 448 F g-1), which was exceptionally related not only to the quality of the synthesis but also the choice of electrode and electrolyte materials. Moreover, each component used in the construction of the hybrid supercapacitor also played a key role in to achieving high capacitance value. The results demonstrated the remarkable performance and stability of the superbat.

Spontaneous Nucleation and Growth of Graphene Flakes on Copper Foil in the Absence of External Carbon Precursor in Chemical Vapor Deposition.

In this work, we uncover a mechanism initiating spontaneous nucleation of graphene flakes on copper foil during the annealing phase of chemical vapor deposition (CVD) process. We demonstrate that the carbon in the bulk of copper foil is the source of nucleation. Although carbon solubility in a pure copper bulk is very low, excess carbon can be embedded inside the copper foil during the foil production process. Using time-of-flight secondary ion mass spectrometry, we measured the distribution profile of carbon atoms inside the copper foils and its variation by thermal annealing. We also studied the role of hydrogen in the segregation of carbon from the bulk to the surface of copper during annealing by scanning electron microscopy and Raman analysis. We found that carbon atoms diffuse out from the copper foil and accumulate on its surface during annealing in the presence of hydrogen. Consequently, graphene crystals can be nucleated and grown while "any external" carbon precursor was entirely avoided. To our knowledge, this is the first time that such growth has been demonstrated to take place. We believe that this finding brings a new insight into the initial nucleation of graphene in the CVD process and helps to achieve reproducible growth recipes.

Foamlike 3D Graphene Coatings for Cooling Systems Involving Phase Change.

Boiling is an efficient heat-transfer mechanism because of the utilization of latent heat of vaporization and has the potential to be used for cooling high-power electronic devices. Surface enhancement is one of the widely used techniques for heat-transfer augmentation in boiling systems. Here, an experimental investigation was conducted on chemical vapor deposition-grown three-dimensional (3D) foamlike graphene-coated silicon surfaces to investigate the effect of pore structures on pool boiling heat transfer and corresponding heat-transfer enhancement mechanisms. 3D graphene-coated samples with four graphene thicknesses were utilized along with a plain surface to investigate boiling heat-transfer characteristics and enhancement mechanisms. A high-speed camera was used to provide a deeper understanding of the bubble dynamics upon departure of emerging bubbles and visualize vapor columns in different boiling regimes. On the basis of the obtained results, in addition to interfacial evaporation, mechanical resonance of the 3D structure had also a considerable effect on vapor column formation. The results indicated that there is an optimum thickness, which exhibits the best performance in terms of boiling heat transfer.

Controlled growth of large area multilayer graphene on copper by chemical vapour deposition.

The growth of nearly full coverage of multilayer graphene on the surface of a 99.8% purity copper foil has been experimentally studied. It has been shown that the film thickness can be controlled by a single parameter, the growth time, and growth can be extended until nearly full coverage of more than one layer graphene over the copper surface. The results are supported by scanning electron microscopy and Raman analysis together with optical transmittance and sheet resistance measurements. It has been verified that silicon oxide impurity particles within the copper act as catalysts and the seeds of multilayer graphene islands. The linear increase of the average thickness of graphene to the growth time has been attributed to the interplay between the mean distance between the impurities on the surface and the molecular mean free path in the process gas. A qualitative model is proposed to explain the microscopic mechanism of the multilayer growth on copper. These results contribute to the understanding of the chemical vapour deposition growth kinetics towards the objective of large area high quality graphene production with tuneable layer thickness.

Note: Controlled fabrication of suspended metallic vacuum tunneling gaps.

We developed a new fabrication technique for the realization of nanogaps using conventional lithography and in situ controlled thermal evaporation. A 20-40 nm gap between two suspended metallic electrodes is shrunk down to about 1 nm using controlled thermal evaporation. It is demonstrated that with this technique rigid and stable metallic vacuum tunneling junctions can be consistently produced. The fabricated nanogaps were characterized by I-V measurements and their gap sizes and barrier heights were interrogated using the Simmons' model.

Tuning of nanogap size in high tensile stress silicon nitride thin films.

High tensile stress suspended structures are demanded for high mechanical quality factor applications. However, high tensile stress causes distortion of the original shapes by contracting, buckling, and bending the suspended structures. We demonstrate a method to compensate for the shape deformation of suspended structures due to intrinsic tensile stress after they are released. With a new design, the distance between two suspended structures after wet etch can easily be tuned by a single fabrication beyond the lithographic resolution limits. The technique is simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability with 2.4 nm standard error.

Absolute negative resistance induced by directional electron-electron scattering in a two-dimensional electron gas.

A three-terminal device formed by two electrostatic barriers crossing an asymmetrically patterned two-dimensional electron gas displays an unusual potential depression at the middle contact, yielding absolute negative resistance. The device displays momentum and current transfer ratios that far exceed unity. The observed reversal of the current or potential in the middle terminal can be interpreted as the analog of Bernoulli's effect in a Fermi liquid. The results are explained by directional scattering of electrons in two dimensions.