arXiv subjects

Joyce E. Coppock

Publications and source records attributed to Joyce E. Coppock.

7 recordsLinked to original sources

arXiv2026

Microfabricated Au and Au/graphene bilayer platelets for levitation experiments

We describe a fabrication process for preparing liquid suspensions of micron-scale Au and Au/graphene bilayer platelets using thin-film deposition, optical lithography, ion milling, hydrofluoric acid (HF) substrate etching, and release from the substrate into a liquid suspension. Residual HF is removed through repeated centrifugation, decanting, and dilution cycles. The resulting suspension is characterized by electrospray deposition onto a secondary substrate, followed by electron and atomic force microscopy. The deposited platelets exhibit minimal aggregation, and the overall platelet yield reaches up to 30% of the platelets originally patterned on the wafer. Lateral force microscopy further confirms that the Au/graphene bilayer remains intact throughout fabrication, release, and electrospray deposition. This process provides a practical route for preparing high-quality platelet suspensions for levitated nanoparticle experiments and other applications requiring suspensions of two-dimensional nanostructures.

cond-mat.mes-hall↗

arXiv2026

Carbon encapsulation of levitated Au nanoparticles

We investigate the formation of a barrier to evaporation that develops when levitated nanoscale Au nanoparticles are exposed to pulses of 532 nm laser radiation in a high vacuum (pressure $p=10^{-8}-10^{-7}$ Torr) environment. Our data are derived from precision measurements of the charge to mass ratio ($Q/M$) of $\sim$200 nm diameter Au particles confined in a quadrupole ion trap. We characterize the development of the barrier over time as the particle is repeatedly heated with laser pulses and determine the impact of variations of the interval between pulses and of exposure to several gases added to the vacuum chamber. We observe a slow increase in the mass of particles upon prolonged exposure to the vacuum, which we attribute to the growth of a barrier layer. For particles that have acquired a barrier during exposure to CO, we observe a rapid decrease in their mass upon subsequent exposure to O$_2$. These findings are consistent with the growth and subsequent oxidation of a graphene layer on the Au that forms the barrier to evaporation. However, we have not found that the rate of formation of the barrier depends on the pressure of carbon-containing gases (CO, C$_2$H$_4$, CO$_2$) we have added to the chamber. We hypothesize that a rare surface state on the solid Au particle catalyzes the reaction that introduces C to the particle. Repeated laser pulse heating is necessary--either to enable diffusion away from this state or to create fresh states that allow continued C uptake--to facilitate the growth of the surface graphene layer.

cond-mat.mes-hall↗

arXiv2024

Focused deposition of levitated nanoscale Au droplets

We describe a method for depositing nanoscale liquid Au droplets, initially levitated in an ion trap in high vacuum, onto a remote substrate. A levitated Au nanosphere is melted, expelled from the trap, and maintained in the molten state with a laser directed along the droplet trajectory until it reaches the substrate and rapidly solidifies. Also during transit, the charged droplets are focused to a small region of the substrate with an electrostatic lens. After deposition, the substrate can be removed from the vacuum chamber and imaged and analyzed by techniques such as electron microscopy and energy dispersive spectroscopy. Over 90% of launched particles are deposited on the substrate, and when the lens is focused, particles land in a region of diameter 120 $μ$m after traversing a distance of 236 mm. Our technique is of value for analysis of materials prepared or processed while levitated that can be melted. Also, Au droplets may be useful as tracers for future experiments involving smaller projectiles or oriented solids.

cond-mat.mes-hall↗

arXiv2017

A dual-trap system for the study of charged rotating graphene nanoplatelets in high vacuum

We discuss the design and implementation of a system for generating charged multilayer graphene nanoplatelets and introducing a nanoplatelet into a quadrupole ion trap in high vacuum. Levitation decouples the platelet from its environment and enables sensitive mechanical and magnetic measurements. The platelets are generated via liquid exfoliation of graphite pellets and charged via electrospray ionization. A single platelet is trapped at a pressure of several hundred millitorr and transferred to a trap in a second chamber, which is pumped to UHV pressures for further study.

physics.ins-det↗

arXiv2016

Optical and magnetic measurements of gyroscopically stabilized graphene nanoplatelets levitated in an ion trap

Using optical measurements, we demonstrate that the rotation of micron-scale graphene nanoplatelets levitated in a quadrupole ion trap in high vacuum can be frequency locked to an applied radio frequency (rf) electric field. Over time, frequency locking stabilizes the nanoplatelet so that its axis of rotation is normal to the nanoplatelet and perpendicular to the rf electric field. We observe that residual slow dynamics of the direction of the axis of rotation in the plane normal to the rf electric field are determined by an applied magnetic field. We present a simple model that accurately describes our observations. From our data and model we can infer both a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation, which we compare to theoretical values. Our results establish that trapping technologies have applications for materials measurements at the nanoscale.

cond-mat.mes-hall↗

arXiv2015

Ambipolar High Mobility Hexagonal Transistors on Hydrogen-Terminated Silicon (111) Surfaces

We have fabricated ambipolar transistors on chemically prepared hydrogen-terminated Si(111) surfaces, in which a two-dimensional electron system (2DES) or a two-dimensional hole system (2DHS) can be populated in the same conduction channel by changing the gate voltage of a global gate applied through a vacuum gap. Depending on the gate bias, ion implanted n$^+$ and p$^+$ regions function either as Ohmic contacts or as in-plane gates, which laterally confine the carriers induced by the global gate. On one device, electron and hole densities of up to $7.8\times10^{11}$ cm$^{-2}$ and $7.6\times10^{11}$ cm$^{-2}$ respectively are obtained. The peak electron mobility is $1.76\times10^5$ cm$^{2}$/Vs, and the peak hole mobility is $9.1\times10^3$ cm$^{2}$/Vs at 300 mK; the ratio of about 20 is mainly due to the very different valley degeneracies (6:1) of electrons and holes on the Si(111) surface. On another device, the peak electron mobility of $2.2\times10^5$ cm$^{2}$/Vs is reached at 300 mK. These devices are hexagonal in order to investigate the underlying symmetry of the 2DESs, which have a sixfold valley degeneracy at zero magnetic field. Three magnetoresistance measurements with threefold rotational symmetry are used to determine the symmetry of the 2DESs at different magnetic field. At filling factor $1 < ν< 2$, the observed anisotropy can be explained by a single valley pair occupancy of composite fermions (CFs). Qualitatively the CFs preserve the valley anisotropy, in addition to the twofold valley degeneracy. At magnetic field up to 35 T, the 2/3 fractional quantum Hall state is observed with a well developed hall plateau; at $ν<2/3$, the three magnetoresistances show a large anisotropy (50:1). We also show that device degradation is not a serious issue for our measurements, if the device is kept in vacuum or a nitrogen gas environment and its time in air is minimized.

cond-mat.str-el↗

arXiv2015

Cooling of Levitated Graphene Nanoplatelets in High Vacuum

We demonstrate cooling of the center of mass motion of charged graphene nanoplatelets levitated in a quadrupole ion trap in high vacuum down to temperatures of 20 K. Parametric feedback based on optical measurements of particle motion was used to achieve the particle cooling at pressure $p<10^{-6}$ Torr, and cooling along all three axes of motion was observed. Dependence of cooling on the electric fields was measured by varying DC voltages on a set of auxiliary electrodes used to spatially shift the trap minimum. Methods to calibrate mass and charge of the nanoplatelet by measuring its motion frequency dependence on discharge were also explored.

cond-mat.mes-hall↗