An understanding of soil properties are of critical importance for optimizing agricultural input use (such as irrigation water and fertilizer) and for general land management strategies. However, obtaining information about soil properties in real-time can be a challenge, which limits management approaches, and can lead to excess input and energy use, reduced profitability, and environmental concerns. Remote imaging can provide high-resolution, but measurements may be infrequent, impacted by weather and plant growth, and could requires inference to determine properties of interest. Installed sensors that directly sample soil can directly provide the desired information directly, but are often bulky and expensive, limiting their use to a small number of sensors per field. This is a concern since many important species of interest (such as soil nitrate), can vary significantly (on the order of 10s of meters), as such, ideally, higher spatial density measurements are needed to capture current conditions and enable optimized management strategies.

In this presentation a number of devices (capacitive, ion-selective, and enzyme/microbe selective) recently developed for real-time, in-situ, high-spatial density monitoring of soil conditions such as moisture and ion (particularly nitrate) concentration, and microbial activity will be discussed. In order to enable broad distribution of large numbers of devices over large areas, these sensors are fabricated using additive printing techniques (such as screen printing) and biodegradable materials for substrates, conductors, encapsulants, stimuli-responsive materials, etc.), so that the sensors degrade harmlessly into the soil when no longer required, enabling large amounts to be used without the need for maintenance or collection or the production of excess waste.

Since the invention of the chip-scale atomic clock in 2001, and its subsequent commercialization in 2011, many research groups and companies worldwide have begun programs to develop similar or related instruments. In this talk, I will present recent work in the Atomic Devices and Instrumentation Group at NIST to develop next-generation devices based on silicon micromachining, atomic spectroscopy and photonics. This will include photonically integrated atomic wavelength references, chip-scale optical clocks and novel atomic diffractive optical elements. I will conclude with a discussion of “NIST on a Chip”, a new effort at NIST to provide low-cost SI calibration at the chip-scale across a range of physical quantities.

Every moving mechanical system consisting of contacting/sliding/rotating contacts ranging from nanoscale switches, micro-machines to large macroscale systems such as wind turbines suffers from the energy loss due to wear and friction and it amounts to roughly a quarter of total energy loss worldwide. There is growing demand to develop advanced coatings and lubricants that can not only reduce the energy loss but also last longer, can work in any environment, don’t need replenishment, cheaper to produce on large scale and most importantly are environment friendly. In this context, I’ll discuss our research efforts, which are focused on understanding the atomic scale origin of the friction and how nanoscale interactions of materials at the sliding interface could be manipulated to have its impact on the macroscale. I’ll review our earlier work on demonstrating diamond-based micro-machines with almost no wear even after millions of cycles of operations as it forms impervious tribolayer after initial run-in and some recent work on utilizing a combination of 2D materials and nanoparticles as a solid lubricant in reducing friction and wear to near zero (superlubricity) in rough steel contacts at macroscale. I’ll discuss the underlying mechanism in both cases and how one can translate these fundamental discoveries into real-world applications by working collaboratively with industry.

I will discuss nanoscale asperity-on-asperity contact and sliding experiments conducted using an in situ nanoindentation apparatus inside a transmission electron microscope (TEM). The instrument has been customized to permit atomic-scale resolution of contact formation, asperity sliding, and adhesive separation of a nanocontact with real-time TEM imaging [1-7], with a new innovation in the instrumentation that allows two AFM tips to be studied in dynamic loaded contact [6, 8]. Forming and separating the contacts without sliding revealed small adhesion forces; sliding during retraction resulted in a nearly 20 times increase in adhesion. These effects were repeatable multiple times. We attribute this surprising sliding-dependent adhesion to the removal of passivating terminal species from the surfaces, followed by re-adsorption of these species after separating the surfaces [8]. Preliminary results from molecular dynamics simulations to elucidate this effect will be discussed. I will also present new results from nanocontact experiments of 2D materials obtained in situ using transmission electron microscopy (TEM). We have observed tip-on-tip contact and sliding behavior at the nanoscale for self-mated contacts of few-layer MoS₂, revealing intrinsic contact, adhesion, and friction properties of these ultrathin layers. I will present results comparing the behavior of nanometer-scale thick MoS₂ layers with different degrees of nanocrystallinity, and discuss collaborative work modeling these experiments using molecular dynamics simulations.

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1. Tribol. Lett. 2013, 50 (1), 81-93. https://doi.org/10.1007/s11249-012-0097-3

1. Adv. Mat. Interf. 2015, 2 (9). https://doi.org/10.1002/admi.201400547

1. Tribol. Lett. 2015, 59 (1), 1 (11 pp.). 10.1007/s11249-015-0539-9

1. MRS Bulletin 2019, 44 (06), 478-486. https://doi.org/10.1557/mrs.2019.122

1. Carbon 2019, 154, 132-139. https://doi.org/10.1016/j.carbon.2019.07.082

1. ACS Appl. Mat. Interf. 2019, 11 (43), 40734-40748. https://doi.org/10.1021/acsami.9b08695

1. Langmuir 2019, 35 (48), 15628-15638. https://doi.org/10.1021/acs.langmuir.9b02029

Time-resolved electric force microscopy (EFM) records a sample's transient photocapacitance by observing the oscillation frequency of a charged microcantilever. When the transient is faster than half a cantilever oscillation period, a few microseconds, the demodulated cantilever frequency cannot be clearly interpreted due to a violation of Bedrosian's product theorem for analytic signals. We have introduced a scanned-probe method for measuring photocapacitance transients in semiconductors that sidesteps this seemingly fundamental limit. In our experiment (doi:10.1126/sciadv.1602951), a voltage pulse is applied to charge the cantilever while a light pulse is applied to generate free charges in the sample. These sample charges shift the cantilever's frequency and phase of oscillation. Snapshots of the sample's evolving photocapacitance are obtained by recording the net change in cantilever phase as a function of the time delay between the light and voltage pulses. We use "phase kick" EFM measurements reveal a biexponential buildup of charge in a polymer-blend solar-cell film, with the fast component having a risetime of 40 microseconds at high light intensity. We demonstrate the superior signal-to-noise and time resolution of pk-EFM by recording the 35 ns probe-wiring time constant of our apparatus in 100 ms of acquisition time.

In support of this work we developed a rigorous treatment of the interaction of a charged cantilever with an electrically conductive sample. The results were surprising. Starting from an electromechanical model of the cantilever-sample interaction, we used Lagrangian mechanics to derive coupled equations of motion for the cantilever position and charge (doi:10.1103/PhysRevApplied.11.064020). A key player in our theory is the transfer function describing the voltage drop across the tip-sample gap; this transfer function depends on the tip-sample capacitance and on the complex sample impedance (i.e., capacitance and resistance). We conclude that cantilever frequency and dissipation measure the real and imaginary parts, respectively, of this transfer function while broadband local dielectric spectroscopy measures the entire transfer function between 0.1 Hz and a few MHz. We find that sample capacitance and resistance both contribute to cantilever frequency shift and dissipation. We have used these insights to study light-induced conductivity and capacitance as a function of time, temperature, and substrate in organic semiconductors and a series of 2D and 3D perovskites. Many perovskite samples show a conductivity that rises promptly upon illumination and recovers slowly in the dark, with an activated temperature dependence consistent with ion or vacancy motion. These findings suggest that light creates vacancies.