We describe apparatus and experimental procedures for high stability precision measurements of levitated nanoscale particles confined in an ion trap in high vacuum. We discuss methods for particle generation and collection using electrospray emission, for rapid characterization by direct imaging of thermal motion, and for transfer of the particle from the trap where it is collected to a separate analysis trap in order to achieve better vacuum and lower noise. In the analysis trap at high vacuum (pressure p≃10^-8 Torr), we employ thermostatic control of the trapped particle oscillation amplitudes, allowing long-term, precision measurements of oscillation frequencies, from which the charge to mass ratio (Q/M) can be deduced. Under these conditions, we achieve Q/M measurement precision approaching 10^-5. This sensitivity will enable, for example, investigations of the surface chemistry of μm-scale levitated materials in ultra-high vacuum environments.

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

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 ∼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_2H_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.

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. During transit, the charged droplets are focused to a small region of the substrate with an electrostatic lens. After deposition, the substrate is 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 mu 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.

We investigate melting and undercooling in nanoscale (radius ∼100 nm) gold particles that are levitated in a quadrupole ion (Paul) trap in a high vacuum environment. The particle is heated via laser illumination and probed using two main methods. Firstly, measurements of its mass are used to determine the evaporation rate during illumination and infer the temperature of the particle. Secondly, direct optical measurements show that the light scattered from the particle is significantly different in its liquid and solid phases. The particle is repeatedly heated across its melting transition, and the dependence of heating behavior on particle size is investigated. Undercooling – the persistence of a liquid state below the melting temperature – is induced via multistage laser pulses. The extent of undercooling is explored and compared to theoretical predictions.

We describe a technique for preparing highly charged Au nanospheres that can be collected in a quadrupole ion trap and heated beyond the melting point of Au, using a laser, without discharging. Au nanospheres are first added to solutions of (NH4)(2)CO3 to prepare stable suspensions. Electrospray emission of these suspensions introduces single Au nanospheres into an ion trap in high vacuum, where their charge and mass can be determined from the effect of discrete discharging on their charge to mass ratio, (q/m). After heat treatment to remove residue, q is stable and the nanosphere temperature T can be estimated by mass loss from Au thermal evaporation. Using this technique, we probe the nanosphere melting behavior across the phase transition at T = 1337 K. Finally, we observe that contaminants in the vacuum, probably C, can have a profound effect on the thermal and optical properties of the Au nanospheres near their melting point. (C) 2021 The Authors. Published by Elsevier Ltd.

Abstract We describe a technique for preparing highly charged Au nanospheres that can be collected in a quadrupole ion trap and heated beyond the melting point of Au, using a laser, without discharging. Au nanospheres are first added to solutions of ( NH 4 ) 2 CO 3 to prepare stable suspensions. Electrospray emission of these suspensions introduces single Au nanospheres into an ion trap in high vacuum, where their charge and mass can be determined from the effect of discrete discharging on their charge to mass ratio, ( q / m ) . After heat treatment to remove residue, q is stable and the nanosphere temperature T can be estimated by mass loss from Au thermal evaporation. Using this technique, we probe the nanosphere melting behavior across the phase transition at T =1337 K. Finally, we observe that contaminants in the vacuum, probably C, can have a profound effect on the thermal and optical properties of the Au nanospheres near their melting point.

Intrinsic Si(111) surfaces passivated with atomic hydrogen are an ideal platform to host two-dimensional electron systems. Traditional methods to probe these surfaces, however, typically involve the placement of dopants and metals directly onto the surface and subsequent high temperature processing, which can be harsh and invasive and lead to surface degradation. Here, we detail a non-invasive gating approach for probing two-dimensional electron systems on intrinsic H-Si(111) surfaces using a silicon-on-insulator (SOI) gating assembly. In this architecture, all harsh device fabrication is performed on a single SOI chip, ensuring that the H-Si(111) surface remains in pristine condition, or as close to the original manufactured intrinsic-Si wafer as possible. To achieve this, we intentionally keep our H-Si(111) surfaces free of any dopants or metals, which are instead placed on the adjacent SOI chip. All electrical components, including Ohmic contacts and accumulation and depletion gates, are housed in the SOI piece. The Ohmic contacts on the SOI piece are brought into physical and electrical contact with the pristine H-Si(111) piece after being van der Waals bonded at room temperature, while all gates on the SOI piece are separated from the H-Si(111) surface by vacuum. Architecture details, baseline operation tests, and 77K device characterization measurements will be discussed, as well as the implications of going beyond H-Si(111) surfaces and using our device architecture to facilitate transport measurements on halogen-terminated Si surfaces.

A charged nanoparticle that is confined and cooled in an ion trap can, in principle, be expelled from the trap and directed onto a substrate with high positional accuracy using an electrostatic lens. This deposition technique could provide a complement to studies of trapped nanoscale objects by allowing examination of the object outside the trap. It may also be used to assemble new types of structures (for example, by depositing a 2D material onto a reactive surface in high vacuum). In our system, a charged nanoparticle held in a quadrupole electric field trap is released from the trap and directed toward a removable indium tin oxide (ITO) coated substrate in ultrahigh vacuum (UHV), using an Einzel lens to focus the particle's trajectory. We have worked with a variety of materials: graphene nanoplatelets around 1 micron in diameter, as well as three-dimensional nanoparticles (including gold, silver, tin, silica, polystyrene, and graphite) with diameters of 200-800 nm. We have consistently detected particles striking the substrate by means of a charge sensor connected to the conductive substrate coating. Some particles, but not others, are observed to stick to the substrate; we are currently working to increase the chance of adhesion for metal nanoparticles by raising their temperature before deposition. We have had some success in locating the deposited particles using a camera positioned above the substrate; efforts to improve the imaging method are ongoing.

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 radiofrequency electric field E-rf. Over time, frequency-locking stabilizes the nanoplatelet so that its axis of rotation is normal to the nanoplatelet and perpendicular to E-rf. We observe that residual slow dynamics of the direction of the axis of rotation in the plane normal to E-rf is 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.

We discuss the design and implementation of a system to generate charged multilayer graphene nanoplatelets and introduce a nanoplatelet into a quadrupole ion trap under vacuum. Levitation decouples the platelet from the 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 ultra high vacuum pressures for further study.

Particle trapping technologies provide the opportunity to study two-dimensional materials that are fully decoupled from substrates. We investigate the dynamics of a rotating micron-scale graphene particle that is levitated in high vacuum in a quadrupole ion trap and probed via optical scattering. The particle is spun to frequencies ranging from hundreds of kHz to above 50 MHz using a circularly polarized laser. We observe phase locking of particle rotation frequency to an applied RF electric field. The rotation frequency can be adjusted by changing the applied field frequency. We discuss prospects for measurements of particle properties enabled by this technique.

We report experiment and theory on an ambipolar gate-controlled Si(111)-vacuum field effect transistor where we study electron and hole (low-temperature 2D) transport in the same device simply by changing the external gate voltage to tune the system from being a 2D electron system at positive gate voltage to a 2D hole system at negative gate voltage. The electron (hole) conductivity manifests strong (moderate) metallic temperature dependence with the conductivity decreasing by a factor of 8 (2) between 0.3 K and 4.2 K with the peak electron mobility (∼18  m2/V s) being roughly 20 times larger than the peak hole mobility (in the same sample). Our theory explains the data well using random phase approximation screening of background Coulomb disorder, establishing that the observed metallicity is a direct consequence of the strong temperature dependence of the effective screened disorder.

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 7.8×10^11 cm^-2 and 7.6×10^11 cm^-2 respectively are obtained. The peak electron mobility is 1.76×10^5 cm^2/Vs, and the peak hole mobility is 9.1×10^3 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 2.2×10^5 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.

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.