Name: all-electronic nanosecond-resolved scanning tunneling microscopy: facilitating the investigation of single dopant charge dynamics
Uploaded: 2018-01-19T14:00:00.000+00:00
Duration: 11 min 33 s
Description: Watch this Scientific Journal Video about All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics at JoVE.com

We would like to thank Martin Cloutier and Mark Salomons for their technical expertise. We also thank NRC, NSERC, and AITF for financial support.

1. Initial Setup of Microscope and Experiments
<ol>
	<li>Begin with an ultrahigh vacuum cryogenic-capable STM and associated control software. Cool the STM to cryogenic temperatures. 
	NOTE: Throughout this report, ultrahigh vacuum refers to systems that achieve &#60;10 x 10-10 Torr. The STM should be cooled to cryogenic temperatures; this is especially important when investigating the charge dynamics of dopants, which are thermally activated at modest temperatures. Other chambers may be at room temperature.</li>
	<li>Ensure that the STM tip is equipped with high-frequency wiring (~500 MHz). 
	NOTE: By using pulse shaping methods, a great increase in the time response of a STM with standard cryogenic temperature coaxial wiring (~20 MHz) has been reported by Grosse et al.31</li>
	<li>Connect an arbitrary function generator with at least two channels to the tip (Figure 2), which will be used to prepare the cycles of voltage pulse pairs used for pump-probe experiments.</li>
	<li>Configure the arbitrary function generator so that the pump and probe voltage pulses are generated independently and summed before being fed into the tip.</li>
	<li>Apply the DC bias voltage used for imaging and conventional spectroscopy to the sample (VDC).</li>
	<li>Connect two radio frequency switches to the arbitrary function generator&#39;s output channels.</li>
	<li>Configure the switches so that the tip will be grounded during STM imaging and conventional spectroscopy, and the effective bias is VDC + Vtip during pump-probe experiments (Figure 4A).</li>
	<li>Collect the tunneling current for all measurements through a preamplifier connected to the sample.</li>
</ol>
2. Preparation of the H-Si(100)-(2x1) Reconstruction
<ol>
	<li>Cleave a sample from a 3-4 m&#937;&#183;cm n-type arsenic doped Si(100) wafer by scratching the back of the wafer with a silicon carbide scriber and gently snapping the sample off the wafer with glass microscope slides.</li>
	<li>Affix the sample to an STM sample holder and introduce it to an ultrahigh vacuum chamber adjacent to the STM chamber.</li>
	<li>Degas the sample by resistively heating it to 600 &#176;C (a pyrometer can be used to monitor the sample&#39;s temperature) and holding it at that temperature for at least 6 h in ultrahigh vacuum. 
	NOTE: The pressure will initially rise as the sample and sample holder degas, but should stabilize near the base pressure (&#60;10-10 Torr) after several hours.</li>
	<li>Allow the sample to cool to room temperature before continuing.</li>
	<li>Degas a tungsten filament in the same chamber as the sample by resistively heating the filament to 1800 &#176;C and waiting for the system to recover to base pressure. Turn off the filament before continuing. 
	NOTE: The sample can remain in the chamber during this step because it is passivated by the native oxide layer at its surface, and any contamination of the sample surface caused from this step will be removed subsequently. The filament&#39;s temperature will have to be calibrated to a specific current/voltage applied to the filament using a pyrometer.</li>
	<li>Remove the oxide from the sample surface by flashing the sample to 900 &#176;C and holding it at that temperature for 10 s before cooling it to room temperature. The pressure will increase several orders of magnitude from the base pressure during the flashing procedure. After each of the flashes found throughout this procedure, wait for the sample to cool to room temperature and the system to recover to base pressure before continuing. 
	NOTE: Flashing is defined within this report as heating and cooling the sample with high ramp rates, on the order of 100 &#176;C/s.</li>
	<li>Progressively flash the sample to higher temperatures while attempting to reach a final flash of 1250 &#176;C. Abort any flash where the pressure rises above 9x10-10 Torr to prevent the sample surface from getting contaminated. Record the voltage/current used to achieve the 1250 &#176;C flash (the light given off by the heated filament in step 2.6 will prevent a pyrometer from giving an accurate reading of the sample&#39;s temperature, and thus this setpoint should be used). On the final flash, determine the voltage/current required to heat the sample to 330 &#176;C as the crystal is cooled, then let the sample cool to room temperature, and let the system recover to base pressure before continuing.</li>
	<li>Leak H2 gas into the chamber at a pressure of 1x10-6 Torr and heat the tungsten filament to 1800 &#176;C. 
	NOTE: This has the effect of cracking the H2 to atmoic hydrogen32.</li>
	<li>Hold the sample in these conditions for 2 min before flashing the sample to 1250 &#176;C, holding it at that temperature for 5 s, and cooling it to 330 &#176;C.</li>
	<li>After 1 min of exposure at 330 &#176;C, simultaneously close the H2 leak valve, turn off the tungsten filament, and let the sample cool to room temperature. 
	NOTE: These high flash temperatures affect the distribution of dopants in the sample. Heating to 1250 &#176;C has been found to induce a ~60 nm dopant depletion region near the sample surface30.</li>
	<li>Verify the sample&#39;s quality by taking STM images of the surface. 
	NOTE: Good samples will have large (&#62;30 nm x 30 nm) terraces with a defect rate of &#60;1% (dangling bonds, adsorbed molecules, adatoms, etc.) and will demonstrate the classic Si(100)-(2x1) reconstruction32, which features dimer rows running antiparallel to one another across step edges (Figure 3B).</li>
</ol>
3. Assessing the Quality of the Pump-probe Pulses at the Tunnel Junction
<ol>
	<li>Approach the STM tip to the sample surface by engaging the current feedback controller with a current setpoint of 50 pA and a sample bias of -1.8 V. 
	NOTE: Under these conditions, the tip is estimated to be &#60;1 nm from the sample surface. The STM tip used in this work was produced by chemically etching polycrystalline tungsten. It was sharpened further using a nitrogen etching procedure, which is well described in Rezeq et al.33.</li>
	<li>Look for an area on the sample surface free from large surface defects by taking large area scans (e.g., 50 nm x 50 nm).</li>
	<li>Position the STM tip over an H-Si on the surface, which appear as the dimer rows in STM images (Figure 3B).</li>
	<li>Turn off the current feedback controller</li>
	<li>Set VDC to -1.0 V, Vpump to -0.5 V, Vprobe to -0.5 V, the width of the pump and probe pulses to 200 ns, and the rise/fall time of the pulses to 2.5 ns (Figure 4A).</li>
	<li>Send a series of trains of pump and probe pulses where the relative delay of the pump and probe is swept from -900 ns to 900 ns.</li>
	<li>Plot the tunneling current as a function of the delay between the pump and probe. It will likely demonstrate strong ringing (oscillations in the tunneling current as a function of the relative delay between the pump and probe pulses, Figure 4B). 
	NOTE: Python and Origin software were used to plot, analyze, and evaluate the data collected for this manuscript.</li>
	<li>Repeat steps 3.1&#8211;3.5, but increase the rise/fall times of the pulses. The ringing will decrease as the rise/fall times are increased. 
	NOTE: It&#39;s desired to eliminate ringing to provide the most accurate spectroscopic results, however, the time-resolution of these techniques is limited to the width of the pulses used. 25 ns rise times were used for this work.</li>
</ol>
4. Time-Resolved Scanning Tunneling Spectroscopy (TR-STS)
<ol>
	<li>Position the STM tip over a silicon DB, which appear as bright protrusions at negative tip-sample biases (Figure 3B).</li>
	<li>Turn off the STM current feedback controller.</li>
	<li>Send a train composed of only the probe pulse with a repetition rate of 25 kHz. Over a series of pulse trains, sweep the bias of the probe pulse over a range of 500 mV from the DC bias of -1.8 V. 
	NOTE: This simple experiment is analogous to conventional STS where the conductance is sampled over a range of biases.
	<ol>
		<li>Configure the duration of the pulse trains (each with a different bias) such that the resulting spectra have a signal to noise ratio &#62;10.</li>
	</ol>
	</li>
	<li>Send a train composed of pump pulses set at a fixed bias (such that VDC + Vpump &#62; Vthr) with a repetition rate of 25 kHz. In these experiments, set the VDC, Vpump, and Vthr to -1.8 V, 500 mV and -2.0 V, respectively. 
	NOTE: Pump pulses can have arbitrarily long durations (1 &#181;s is typically sufficient).</li>
	<li>Send a train composed of the pump pulses with the probe pulses followed by a delay of 10 ns. In these experiments, set the amplitude of the pump pulse as 500 mV and of the probe pulse sweep from 50 to 500 mV. 
	NOTE: In this experiment, the probe pulse is sampling the state prepared by the pump pulse, rather than the equilibrium state sampled in conventional STS.
	<ol>
		<li>Subtract the signal obtained when only the pump pulse was applied when displaying/evaluating the signal collected from this step.</li>
	</ol>
	</li>
	<li>Compare the probe only and the pump + probe signals by plotting them in the same graph. Any hysteresis in the two signals is an indication of dynamics that can be probed with time-resolved STM techniques. 
	NOTE: By keeping the range of the probe pulse fixed and coarsely scanning the DC offset (in 0.25 V steps, for example), one can efficiently map the entire energy range of the sample to identify dynamics accessible to the technique. Pulse durations can be modified depending on the experiment. The width of the pump pulse needs to be longer than the rate at which the dopant is ionized, such that it consistently ionizes the dopant. In general, probe durations should be of the same order as the dynamical process under study, such that the maximum signal can be measured without sampling an average of the two conductance states. When searching for energies at which dynamics exist, it is recommended that durations of the probe are minimized such that only one state of the system is measured to enhance hysteresis. As the relaxation time constants are found, the duration of the probe pulse can be increased to improve the signal to noise ratio.</li>
</ol>
5. Time-resolved STM Measurements of Relaxation Dynamics
<ol>
	<li>Position the STM tip over a silicon DB and turn off the STM current feedback controller.</li>
	<li>Send a train composed of pump pulses set at a fixed bias (such that VDC + Vpump &#62; Vthr) with a repetition rate of 25 kHz. In these experiments, set the VDC, Vpump, and Vthr to -1.8 V, 400 mV and -2.0 V, respectively. 
	NOTE: Pump pulses can have arbitrarily long durations (1 &#181;s is typically sufficient).</li>
	<li>Send a train of pump and probe pulses. Ensure that the probe pulses have an amplitude smaller than the pumps and comparable to the range at which hysteresis occurs (Vprobe &#60; Vpump, Vprobe + VDC&#8776;Vhystersis).</li>
	<li>Sweep the delay between the pump and probe pulse up to several tens of &#181;s.</li>
	<li>Subtract the signal obtained when only the pump pulse was applied. In these experiments, set the VDC, Vpump, and Vprobe to -1.8 V, 400 mV and 210 mV, respectively. Set the relative delay sweep from -5 &#956;s to 35 &#956;s. 
	NOTE: If the signal obtained from the previous step is well fit (R2 &#62; 0.80) by a single exponential decay function, then the lifetime of the transient state prepared by the pump pulse can be extracted from the fit.</li>
</ol>
6. Time-resolved STM Measurements of Excitation Dynamics
<ol>
	<li>Send a train composed of pump pulses set at a fixed bias (such that greater than VDC + Vpump &#62; Vthr) with a repetition rate of 25 kHz. In these experiments, set the VDC and Vthr to -1.8 V and -2.0 V, respectively. Set Vpump between 220 and 450 mV.</li>
	<li>Sweep the duration of the pump pulse from several nanoseconds to several hundred nanoseconds.</li>
	<li>Send a train of pump and probe pulses. The probe pulses should have an amplitude smaller than the pumps and comparable to the range at which hysteresis occurs (Vprobe &#60; Vpump, Vprobe + VDC&#8776;Vhystersis). In these experiments, set Vprobe to 210 mV.</li>
	<li>Subtract the signal obtained when only the probe pulse was applied. 
	NOTE: If the signal obtained is exponential, it indicates that the pump pulse is preparing the transient state (ionized dopant) at a rate that can be extracted from the fit (R2&#62;0.80). The protocol described above is specific to the experiments and equipment described herein. There are many potential avenues for readers to customize their own experimental setup for studies of other systems. For example, the general techniques are not limited to cryogenically cooled STMs; any tip material can be used, and they do not require nitrogen etching. Furthermore, a suitably programmed arbitrary function generator could be used to generate double-pulse waveforms, which would negate the need to sum two independent channels. Lastly, lower bandwidth cabling could be used31.</li>
</ol>

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The authors declare that they have no competing financial interests.

The variant of TR-STS in which the pump pulse is not applied is comparable to conventional STS, except that the system is being sampled at a high frequency rather than continuously. If the durations of the probe pulses are appropriate (&#62;&#915;LH), the TR-STS signal acquired without the pump pulse can be multiplied by a constant proportional to the experiment&#39;s duty cycle to coincide exactly with a conventional STS measurement. This is only possible because the measurements are made without the...

Scanning tunneling microscopy (STM) has become the premier tool in nanoscience for its ability to resolve atomic-scale topography and electronic structure. One limitation of conventional STM, however, is that its temporal resolution is restricted to the millisecond timescale because of the limited bandwidth of the current preamplifier1. It has long been a goal to extend STM&#39;s temporal resolution to the scales on which atomic processes commonly occur. The pioneering work in time-resolved scanning tunneling microscopy (TR-STM) by Freeman et. al.1 utilized photoconductive switches and microstrip transmission lines patterned on the sample to transmit picosecond voltage pulses to the tunnel junction. This junction-mixing technique has been used to achieve simultaneous resolutions of 1 nm and 20 ps2, but it has never been widely adopted due to the requirement of using specialized sample structures. Fortunately, the fundamental insight gained from these works can be generalized to many time-resolved techniques; even though the bandwidth of the STM&#39;s circuitry is limited to several kilohertz, the non-linear I(V) response in STM allows faster dynamics to be probed by measuring the average tunnel current obtained over many pump-probe cycles. In the intervening years, many approaches have been explored, the most popular of which are briefly reviewed below.
Shaken-pulse-pair-excited (SPPX) STM takes advantage of the advancements in ultrafast pulsed laser technologies to achieve sub-picosecond resolution by directly illuminating the tunnel junction and exciting carriers in the sample3. Incident laser light creates free carriers that transiently enhance conduction, and modulation of the delay between the pump and probe (td) allows dI/dtd to be measured with a lock-in amplifier. Because the delay between the pump and probe is modulated rather than the laser&#39;s intensity, as in many other optical approaches, SPPX-STM avoids photo illumination-induced thermal expansion of the tip3. More recent extensions of this approach have extended the timescales over which SPPX-STM can be used to investigate dynamics by utilizing pulse-picking techniques to increase the range of pump-probe delay times4. Importantly, this recent development also provides the ability to measure I(td) curves directly rather than via numerical integration. Recent applications of SPPX-STM have included the study of carrier recombination in single-(Mn, Fe)/GaAs(110) structures5 and donor dynamics in GaAs6. Applications of SPPX-STM face some restrictions. The signal SPPX-STM measures depends on free carriers excited by the optical pulses and is best suited to semiconductors. Additionally, although the tunneling current is localized to the tip, because a large area is excited by the optical pulses, the signal is a convolution of the local properties and material transport. Finally, the bias at the junction is fixed at the measurement timescale so that the dynamics under study must be photoinduced.
A more recent optical technique, terahertz STM (THz-STM), couples free-space THz pulses focused on the junction to the STM tip. Unlike in SPPX-STM, the coupled pulses behave as fast voltage pulses allowing for the investigation of electronically driven excitations with sub-picosecond resolution7. Interestingly, the rectified current generated from the THz pulses results in extreme peak current densities not accessible by conventional STM8,9. The technique has been used recently to study hot electrons in Si(111)-(7x7)9 and image the vibration of a single pentacene molecule10. THz-pulses naturally couple to the tip, however, the necessity to integrate a THz source to an STM experiment is likely to be challenging to many experimenters. This motivates the development of other widely applicable and easily implementable techniques.
In 2010, Loth et al.11 developed an all-electronic technique where nanosecond voltage pulses applied on top of a DC offset electronically pump and probe the system11. The introduction of this technique offered a critical demonstration of unambiguous and practical applications of time-resolved STM to measure previously unobserved physics. Although it is not as fast as junction mixing STM, which preceded it, applying microwave pulses to the STM tip permits arbitrary samples to be investigated. This technique does not require any complicated optical methodologies or optical access to the STM junction. This makes it the easiest technique to adapt to low temperature STMs. The first demonstration of these techniques was applied to the study of spin-dynamics where a spin-polarized STM was used to measure the relaxation dynamics of spin-states excited by the pump pulses11. Until recently, its application remained limited to magnetic adatom systems12,13,14 but has since been extended to the study of carrier capture rate from a discrete mid-gap state15 and charge dynamics of single arsenic dopants in silicon15,16. The latter study is the focus of this work.
Studies on the properties of single dopants in semiconductors have recently attracted significant attention because complementary metal oxide semiconductor (CMOS) devices are now entering the regime where single dopants can affect device properties17. In addition, several studies have demonstrated that single dopants can serve as the fundamental component of future devices, for example as qubits for quantum computation18 and quantum memory19, and as single atom transistors20,15. Future devices may also incorporate other atomic-scale defects, such as the silicon dangling bond (DB) which can be patterned with atomic precision with STM lithography21. To this end, DBs have been proposed as charge qubits22, quantum dots for quantum cellular automata architectures23,24, and atomic wires25,26 and have been patterned to create quantum Hamiltonian logic gates27 and artificial molecules28,29. Moving forward, devices may incorporate both single dopants and DBs. This is an attractive strategy because DBs are surface defects that can easily be characterized with STM and used as a handle to characterize single dopant devices. As an example of this strategy, DBs are used in this work as charge sensors to infer the charging dynamics of near-surface dopants. These dynamics are captured with the use of an all-electronic approach to TR-STM that is adapted from the techniques developed by Loth et al.11
Measurements are performed on selected DBs on a hydrogen terminated Si(100)-(2x1) surface. A dopant depletion region extending approximately 60 nm below the surface, created via thermal treatment of the crystal30, decouples the DB and the few remaining near-surface dopants from the bulk bands. STM studies of DBs have found that their conductance is dependent on global sample parameters, such as the concentration of dopants and the temperature, but individual DBs also show strong variations depending on their local environment16. During an STM measurement over a single DB, the current flow is governed by the rate at which electrons can tunnel from the bulk to the DB (&#915;bulk) and from the DB to the tip (&#915;tip) (Figure 1). However, because the conduction of the DB is sensitive to its local environment, the charge state of nearby dopants influences &#915;bulk (Figure 1B), which can be inferred by monitoring the DB&#39;s conductance. As a result, the conductance of a DB can be used to sense the charge states of nearby dopants, and can be used to determine the rates at which the dopants are supplied electrons from the bulk (&#915;LH) and lose them to the STM tip (&#915;HL). To resolve these dynamics, TR-STS is performed around the threshold voltages (Vthr) at which the tip induces ionization of near-surface dopants. The role of the pump and probe pulses is the same in the three time-resolved experimental techniques presented here. The pump transiently brings the bias level from below to above Vthr, which induces dopant ionization. This increases the conductance of the DB, which is sampled by the probe pulse which follows at a lower bias.
The techniques described in this paper will benefit those wishing to characterize dynamics occurring on the millisecond to nanosecond timescale with STM. While these techniques are not limited to studying charge dynamics, it&#39;s crucial that the dynamics are manifested via transient changes in the conductance of states that can be probed by STM (i.e., states on or near the surface). If the conductance of the transient states does not differ significantly from the equilibrium state, such that the difference between the transient and equilibrium currents multiplied by the probe pulse duty cycle is smaller than the systems noise floor (typically 1 pA), the signal will be lost in the noise and will not be detectable by this technique. Because the experimental modifications of commercially available STM systems required to perform the techniques described in this paper are modest, it&#39;s anticipated these techniques will be widely accessible to the community.

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all-electronic nanosecond-resolved scanning tunneling microscopy: facilitating the investigation of single dopant charge dynamics

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

The results presented in this section of the text have been previously published15,16. Figure 3 illustrates the behavior of an example selected DB with conventional STM. A conventional I(V) measurement (Figure 3A) clearly depicts a sharp change in the conductance of the DB at Vthr = -2.0 V. This behavior is also observed in STM images taken at -2.1 V (...

Watch this Scientific Journal Video about All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics at JoVE.com

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

We demonstrate an all-electronic method to observe nanosecond-resolved charge dynamics of dopant atoms in silicon with a scanning tunneling microscope.

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new ...

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9.5K Views.  University of Alberta. We demonstrate an all-electronic method to observe nanosecond-resolved charge dynamics of dopant atoms in silicon with a scanning tunneling microscope.

Video: All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material1,2,3. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface.
As in other electric-field-sensitive scanned-probe techniques4, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants1,2. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz&frac12; at 0.3 K 5.

Scanning-probe single-electron capacitance spectroscopy facilitates the study of single-electron motion in localized subsurface regions. A sensitive charge-detection circuit is incorporated into a cryogenic scanning probe microscope to investigate small systems of dopant atoms beneath the surface of semiconductor samples.

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Silicon Metal-oxide-semiconductor Quantum Dots for Single-electron Pumping

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by quantum mechanical effects such as tunneling through single dopants, scattering via interface defects, and discrete trap charge states. However, progress in silicon technology has shown that these phenomena can be harnessed and exploited for a new class of quantum-based electronics. Among others, multi-layer-gated silicon metal-oxide-semiconductor (MOS) technology can be used to control single charge or spin confined in electrostatically-defined quantum dots (QD). These QD-based devices are an excellent platform for quantum computing applications and, recently, it has been demonstrated that they can also be used as single-electron pumps, which are accurate sources of quantized current for metrological purposes. Here, we discuss in detail the fabrication protocol for silicon MOS QDs which is relevant to both quantum computing and quantum metrology applications. Moreover, we describe characterization methods to test the integrity of the devices after fabrication. Finally, we give a brief description of the measurement set-up used for charge pumping experiments and show representative results of electric current quantization.

The fabrication process and experimental characterization techniques relevant to single-electron pumps based on silicon metal-oxide-semiconductor quantum dots are discussed.

As mass-produced silicon transistors have reached the nano-scale, their behavior and performances are increasingly affected, and often deteriorated, by ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-&#197;ngstr&#246;m spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.

Here, we present a protocol to investigate the structure and dynamics of interfacial water at the atomic scale, in terms of submolecular resolution imaging, molecular manipulation, and single-bond vibrational spectroscopy.

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal ...

Chemistry

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

Quantitative Atomic-Site Analysis of Functional Dopants/Point Defects in Crystalline Materials by Electron-Channeling-Enhanced Microanalysis

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident high-energy electron beam is rocked with the submicrometric pivot point fixed on a specimen. This method enables us to quantitatively derive the site occupancies and site-dependent chemical information of impurities or intentionally doped functional elements in a specimen, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy attached to a scanning transmission electron microscope, which is of significant interest to current materials science, particularly related to nanotechnologies. This scheme is applicable to any combination of elements even when the conventional Rietveld analysis by X-ray or neutron diffraction occasionally fails to provide the desired results because of limited sample sizes and close scattering factors of neighboring elements in the periodic table. In this methodological article, we demonstrate the basic experimental procedure and analysis method of the present beam-rocking microanalysis.

We provide a general outline of quantitative microanalysis methods for estimating the site occupancies of impurities and their chemical states by taking advantage of electron-channeling phenomena under incident electron beam-rocking conditions, which reliably extract information from minority species, light elements, oxygen vacancies, and other point/line/planar defects.

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident ...

﻿Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Author Spotlight: Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy  

The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This ...

Probing Structural and Dynamic Properties of Trafficking Subcellular Nanostructures by Spatiotemporal Fluctuation Spectroscopy

Imaging-derived mean square displacement (iMSD) is used to address the structural and dynamic properties of subcellular nanostructures, such as vesicles involved in the endo/exocytotic trafficking of solutes and biomolecules. iMSD relies on standard time-lapse imaging, is compatible with any optical setup, and does not need to dwell on single objects to extract trajectories. From each iMSD trace, a unique triplet of average structural and dynamic parameters (i.e., size, local diffusivity, anomalous coefficient) is calculated and combined to build the "iMSD signature" of the nanostructure under study. 
The potency of this approach is proved here with the exemplary case of macropinosomes. These vesicles evolve in time, changing their average size, number, and dynamic properties passing from early to late stages of intracellular trafficking. As a control, insulin secretory granules (ISGs) are used as a reference for subcellular structures that live in a stationary state in which the average structural and dynamic properties of the whole population of objects are invariant in time. The iMSD analysis highlights these peculiar features quantitatively and paves the way to similar applications at the sub-cellular level, both in the physiological and pathological states.

Imaging-derived mean square displacement (iMSD) analysis is applied to macropinosomes to highlight their intrinsic time-evolving nature in terms of structural and dynamic properties. Macropinosomes are then compared to insulin secretory granules (ISGs) as a reference for subcellular structures with time-invariant average structural/dynamic properties.

Imaging-derived mean square displacement (iMSD) is used to address the structural and dynamic properties of subcellular nanostructures, such as vesicles ...

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

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