The authors gratefully acknowledge the financial support provided by North Carolina State University. Milad Abolhasani and Robert W. Epps gratefully acknowledge financial support from the UNC Research Opportunities Initiative (UNC-ROI) grant.

1. Reactor Assembly
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57666/57666fig1.jpg" /> 
Figure 1. Step-by-step illustration of a sample platform assembly process. The panels shows a step-by-step illustration of a sample platform assembly process detailing (i) the initial arrangement of the translation stage and optical post holders on the mounting bread broad, (ii) the mounting of the precursor tube mounting stage and the flow cell onto optical posts, (iii) the attachment of the microfluidic tubing to the custom cross-junction which is under transparency to reveal flow pathways, (iv) the securing of the precursor tubing while simultaneously positioning the first sampling unit, (v) the subsequent connection of additional sampling units with the reactor tubing run through each module, (vi) the tubing pathway of the reactor extension units, and (vii) the securing of the final sampling unit to support the structure and optical posts. <a href="/files/ftp_upload/57666/57666fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Note: Due to the wide array of possible configurations, the exact assembly process of the microfluidic platform might vary; however, the general methods are the same for all arrangements. Detailed below and in Figure 1 is the platform assembly process for a two-precursor, multi-phase flow format with a single extension unit after the 14th sampling port.
<ol>
	<li>Secure the translation stage and post holders to the optical breadboard. Connect the junction mounting stage and flow cell to the posts and fasten them to the platform.</li>
	<li>Wire the reactor tubing and precursor feed lines to the custom cross-junction and feed the tubing through the channels in the raised stage. Ensure that each tubing segment is cut to a length that may comfortably reach the respective syringe pump or collection vial.</li>
	<li>Connect the first sampling port unit to the junction stage and fasten the precursor line cover to the raised mounting stage, securing all the tubular components and the first sampling unit in place.</li>
	<li>Add additional modular reactor units by running the reactor tubing through the desired component and connecting the segments to the rest of the assembled structure. Build out the units from the junction until the desired length and arrangement is obtained. 
	NOTE: The reactor tubing should fit firmly within each unit. Deformations in the tubing (stretching, crimping, etc.) significantly affect the optical signal strength.</li>
	<li>Fasten the support structure to the outlet of the last sampling segment and secure the support to the optical posts connected to the breadboard.</li>
	<li>Connect the feed lines of the flow system to the computer-controlled syringe pumps and feed the reactor outlet into a nitrogen gas pressurized (~12 psig) collection vial.</li>
	<li>Connect 3 fiber optic patch cords to the 3 flow cell ports, and attach the opposite ends to the spectrometer, LED, and deuterium-halogen (DH) light source respectively. Ensure that the cables are able to move smoothly with the full length of the translation stage and without any unnecessary strain on their connection with the flow cell to complete platform assembly (as shown in Figure 2).</li>
</ol>
2. Precursor Preparation
NOTE: The reaction screening system may be applied to the synthesis of various colloidal semiconductor nanocrystals; however, for the purpose of platform development and validation, a CsPbBr3 perovskite synthesis, adapted from Wei et al.6 to better suit flow analyses, was used as a case study reaction. The precursor preparation process is detailed below.
<ol>
	<li>Prepare 15 mL of 0.013 M bromide precursor by combining 109 mg of tetraoctylammonium bromide, 1 mL of oleic acid, and 14 mL of toluene in a sealed 20-mL vial.</li>
	<li>Stir the mixture vigorously at room temperature until a clear solution is obtained.</li>
	<li>Prepare 48 mL of 0.0021 M cesium-lead precursor by first combining 0.6 mmol of cesium hydroxide, 0.6 mmol of lead(II) oxide, and 3 mL of oleic acid in a sealed 8-mL vial with a septum.
	<ol>
		<li>Pierce the septum with a needle for ventilation and heat the solution at 160 &#176;C in an oil bath and vigorously stir it until a clear solution forms (approximately 15 minutes).</li>
		<li>Move the vial and needle to an oven and heat them at 120 &#176;C for 1 h, then remove the venting needle and allow the solution to cool to room temperature in open air.</li>
	</ol>
	</li>
	<li>Add 0.5 mL of the high concentration cesium-lead mixture to 47.5 mL of toluene in a sealed 50-mL vial and stir vigorously.</li>
	<li>Load the bromide precursor and dilute the cesium-lead precursor into their respective syringes and begin the automated characterization process by flowing the two precursors together at the desired conditions (see step 3). 
	NOTE: For the experiments detailed under Representative Results, volumetric injection ratios of 6.4:1 of cesium-lead to bromide and 1:1 of gas carrier phase to net liquid were used at variable total flow rates.</li>
</ol>
3. Interface Operation
Note: The entirety of the data collection is carried out through the automated reaction platform after the user specifies a series of flow conditions to be tested. The general procedures for operating the user interface during this initial input period are detailed below.
<ol>
	<li>Open the automated operation software to view the user interface front panel (shown in Figure 3).</li>
	<li>Move to the Spectrometer settings panel and begin filling all inputs.
	<ol>
		<li>Paste the file path for the desired data saving folder in the File root for data box.</li>
		<li>Under Spectrometer VISA, select the USB connection address for the spectrometer. If the spectrometer USB address is not known, identify its location through the desktop&#39;s Device Manager page.</li>
		<li>Select the integration time, the number of spectra to average per sample, and the number of spectra to save per condition for both absorption (Abs) and fluorescence (Flu). In the case of the synthesis detailed in step 2, set an integration time of 12 ms for the absorption and 4 ms for the fluorescence averaged over 10 spectra.</li>
		<li>If characterizing single-phase flow, move on to the next step, leaving the Multiphase button off. If characterizing multi-phase flow, select the Multiphase button and set the minimum sample length so that approximately 2 complete gas-liquid oscillations may pass the sampling point. Then assign the number of samples to take within that sampling window. 
		NOTE: The sampling time-resolution within a multi-phase flow may be limited by the spectrometer settings, and adjustments may be needed if seeking a higher resolution.</li>
	</ol>
	</li>
	<li>Move to the Pump configuration panel and begin filling all inputs.
	<ol>
		<li>Under Syringe 1 COM, Syringe 2 COM, and Dual Pump COM, assign USB communication addresses to all pumps. See step 3.2.2 for the address identification process.</li>
		<li>Set the syringe inner diameters, which may be found on the pump interface or within the syringe manuals, for all syringes in use. For configurations not implementing all syringes, leave the extraneous syringe diameters at the default values.</li>
		<li>If collecting absorption reference spectra, identify an acceptable blank solution, load it into an attached syringe, and set the respective syringe to a moderate flow rate (approximately 300 &#181;L/min) in the respective Ref flow rate box.</li>
	</ol>
	</li>
	<li>Move to the System configuration panel and begin filling all inputs.
	<ol>
		<li>If the stage locations have been optimized and are properly displayed under Stage positions, select the Use previous position button and move to the next step. If the stage locations have not been optimized, assign a stage position window size and link the approximate stage positions (within the position range) using a .csv vector and the File path box. Leave the Stage increment box at 0.5 mm and the Startup Passes box at 8.</li>
		<li>Calculate the volume of the reactor segment from the center of the junction to the final sampling port and input that value into the System volume box. Leave the Minimum equilibrium time at 10 s.</li>
	</ol>
	</li>
	<li>Double-check the accuracy of all inputs and select the Run button in the top left of the interface. 
	NOTE: Inputs on the front panel may not be changed once the software has begun its operation.</li>
	<li>In the Save reference spectra window, select Yes if the reference spectra will be saved or No if they will not.</li>
	<li>In the Set condition flow rates window, select up to 30 desired flow rate configurations to test, leaving all unused syringe inputs blank.</li>
	<li>Select OK and allow the system to run until all desired conditions have been sampled; the system shuts down on its own. If the system needs to be stopped for any reason, select the Stop early button and allow the process to shut down. 
	CAUTION: Using the Abort button on the top left of the interface will not allow the system to shut down pumps or light sources, potentially damaging equipment and/or posing a significant health risk via UV light exposure.</li>
</ol>
4. Pathlength Corrections
<ol>
	<li>To obtain the pathlength correction correlation for each port, first inject a stable solution of perovskites dispersed in toluene into the reactor segment until the reactor tubing is uniformly filled.</li>
	<li>Run the automated sampling process on this uniform solution for 4 full passes of the flow cell (see step 3 for the operation procedure).</li>
	<li>Apply baseline corrections for both fluorescence and absorption spectra, then average overall for the passes by port location. Normalize all curves in reference to the wavelength intensities at 455 nm and 485 nm for absorption and fluorescence respectively (see Figure 4).</li>
	<li>Use the normalization factor calculated at each port to proportionally scale the curves of all subsequent spectra.</li>
</ol>

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North Carolina State University has filed a provisional patent (#62/558,155) on the discussed microfluidic platform.

Automated sampling system: The autonomous operation of the screening platform is carried out with a central control finite state machine. Movement between these states occurs sequentially with multiple recursive segments to allow for operation across a varying number of sampling conditions. The general system controls can be divided into 3 core stages. First, the system begins with an initialization step, which establishes communications through each USB-controlled component, automatically defines file s...

The advent of semiconductor nanocrystals, particularly quantum dots, has driven significant advancements in electronic materials research and manufacturing. For example, quantum dot LEDs1 have already been implemented in commercially available &#34;QLED&#34; displays. More recently among this class of semiconductors, perovskites have sparked substantial interest and research towards high-efficiency and low-cost PV technologies. Since the first demonstration of a perovskite-based PV in 2009,2 the lab-scale power conversion efficiency of perovskite-based solar cells has increased at a rate unparalleled by any PV technology in history.3,4 In addition to the driving interest in perovskite-based PVs, a variety of recent methods describing the facile colloidal synthesis of perovskite nanocrystals have created the opportunity for low-cost, solution-phase processing of perovskite QDs in commercial electronics.5,6,7,8,9,10,11,12,13,14
In the effort towards large-scale nanomanufacturing of colloidal perovskite QDs, a better fundamental understanding of the nanocrystal growth pathways and an effective control of the reaction conditions must first be developed. However, existing studies of these processes have traditionally relied on flask-based approaches. Batch synthesis strategies present a variety of inherent limitations in terms of material characterization and production, but most significantly, flask-based techniques are highly inefficient in screening time and precursor consumption, and demonstrate flask size-dependent mass transfer properties, which inhibit the synthesis consistency.15 To effectively study the growth pathways of colloidal semiconductor nanocrystals across the large variety of reported syntheses procedures and within the broad relevant sample space, a more efficient screening technique is required. Over the past two decades, a range of microfluidic strategies have been developed for studies of colloidal nanocrystals leveraging the substantially lower chemical consumption, the accessibility of high-throughput screening methods, and the potential for a process control implementation in continuous synthesis systems.12,16,17,18,19,20
In this work, we report the design and development of an automated microfluidic platform for the high-throughput in situ studies of colloidal semiconductor nanocrystals. A novel translating flow cell, a highly modular design, and the integration of off-the-shelf tubular reactors and fluidic connections form a unique and adaptable reconfigurable platform with direct applications in the discovery, screening, and optimization of colloidal nanocrystals. Capitalizing on the translational capability of our detection technique (i.e., a three-port flow cell), for the first time, we demonstrate the systematic decoupling of mixing and reaction timescales, while simultaneously improving the sampling efficiency and collection rates over traditional stationary flow cell approaches. The utilization of this platform enables the high-throughput and precise band-gap engineering of colloidal nanocrystal syntheses towards continuous nanomanufacturing strategies.

<table border="0" cellpadding="0" cellspacing="0" style="width:888px;" width="888">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Toluene</td>
			<td style="width:191px;">Fisher Scientific</td>
			<td style="width:168px;">AC364410010</td>
			<td style="width:257px;">99.85% extra over molecular sieves</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Oleic acid</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">364525&#160;ALDRICH</td>
			<td>technical grade 90%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Cesium hydroxide (50 wt% in water)</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">232041 ALDRICH</td>
			<td>50 wt% in water &#62; 99.9% trace metals</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Lead(II) oxide</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">211907 SIGMA-ALDRICH</td>
			<td>&#62; 99.9% trace metals basis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Tetraoctylammonium bromide</td>
			<td style="width:191px;">Sigma Aldrich</td>
			<td style="width:168px;">294136 ALDRICH</td>
			<td>98%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/16&#34; OD, 0.04&#34; ID FEP tubing</td>
			<td style="width:191px;">MicroSolv</td>
			<td style="width:168px;">48410-40</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/16&#34; OD, 0.02&#34; ID ETFE tubing</td>
			<td style="width:191px;">MicroSolv</td>
			<td style="width:168px;">48510-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">0.02&#34; thru hole PEEK Tee</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-712</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/4-28 ETFE flangeless ferrule for 1/16&#34;</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-200N</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">1/4-28 PEEK flangeless nut for 1/16&#34;</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">P-230</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">4-way PEEK L-valve</td>
			<td style="width:191px;">IDEX Health &#38; Science</td>
			<td style="width:168px;">V-100L</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Syringe pump</td>
			<td style="width:191px;">Harvard Apparatus</td>
			<td style="width:168px;">70-3007</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">8 mL stainless steel syringe</td>
			<td style="width:191px;">Harvard Apparatus</td>
			<td style="width:168px;">70-2267</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">25 mL glass syringe</td>
			<td style="width:191px;">Scientific Glass Engineering</td>
			<td style="width:168px;">25MDF-LL-GT</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical breadboard</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">MB1224</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">300 mm translation stage</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">LTS300</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical post</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">TR2-4</td>
			<td>TR2, TR3, or TR4</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Optical post holder</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">PH4-6</td>
			<td>PH4 or PH6</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">365 nm LED</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">M365LP1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">LED driver</td>
			<td style="width:191px;">ThorLabs</td>
			<td style="width:168px;">LEDD1B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">600 micron patch cord</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">QP600-1-SR</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Deuterium-halogen light source</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">DH-2000-BAL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Miniature spectrometer</td>
			<td style="width:191px;">Ocean Optics</td>
			<td style="width:168px;">FLAME-S-XR1-ES</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Multifuction I/O device (DAQ)</td>
			<td style="width:191px;">National Instruments</td>
			<td style="width:168px;">USB-6001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Virtual Instrument Software</td>
			<td style="width:191px;">National Instruments</td>
			<td style="width:168px;">LabVIEW 2015 SP1</td>
			<td></td>
		</tr>
	</tbody>
</table>

a modular microfluidic technology for systematic studies of colloidal semiconductor nanocrystals

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light emitting diodes (LEDs) and photovoltaics (PVs). Among this material group, inorganic/organic perovskites have demonstrated significant improvement and potential towards high-efficiency, low-cost PV fabrication due to their high charge carrier mobilities and lifetimes. Despite the opportunities for perovskite QDs in large-scale PV and LED applications, the lack of fundamental and comprehensive understanding of their growth pathways has inhibited their adaptation within continuous nanomanufacturing strategies. Traditional flask-based screening approaches are generally expensive, labor-intensive, and imprecise for effectively characterizing the broad parameter space and synthesis variety relevant to colloidal QD reactions. In this work, a fully autonomous microfluidic platform is developed to systematically study the large parameter space associated with the colloidal synthesis of nanocrystals in a continuous flow format. Through the application of a novel translating three-port flow cell and modular reactor extension units, the system may rapidly collect fluorescence and absorption spectra across reactor lengths ranging 3 - 196 cm. The adjustable reactor length not only decouples the residence time from the velocity-dependent mass transfer, it also substantially improves the sampling rates and chemical consumption due to the characterization of 40 unique spectra within a single equilibrated system. Sample rates may reach up to 30,000 unique spectra per day, and the conditions cover 4 orders of magnitude in residence times ranging 100 ms - 17 min. Further applications of this system would substantially improve the rate and precision of the material discovery and screening in future studies. Detailed within this report are the system materials and assembly protocols with a general description of the automated sampling software and offline data processing.

Sample spectra:&#160;Utilizing the discussed microfluidic platform, the nucleation and growth stages of colloidal semiconductor nanocrystals at the synthesis temperature can be directly studied by monitoring the time-evolution of the absorption and fluorescence spectra of the formed nanocrystals under uniform mixing conditions. Figure 5A shows an example set of spectra obtained within a single pass of the three-port flow cell...

Watch this Scientific Journal Video about A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals at JoVE.com

A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light ...

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Research

JoVE Journal

Engineering

9.5K Views.  North Carolina State University. Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Video: A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

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 ...

Scalable Nanohelices for Predictive Studies and Enhanced 3D Visualization

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing applications.&#160; For predictive simulations, it has become increasingly important to be able to model the structure of nanohelices accurately.&#160; To study the effect of local structure on the properties of these complex geometries one must develop realistic models.&#160; To date, software packages are rather limited in creating atomistic helical models.&#160; This work focuses on producing atomistic models of silica glass (SiO2) nanoribbons and nanosprings for molecular dynamics (MD) simulations. Using an MD model of &#8220;bulk&#8221; silica glass, two computational procedures to precisely create the shape of nanoribbons and nanosprings are presented.&#160; The first method employs the AWK programming language and open-source software to effectively carve various shapes of silica nanoribbons from the initial bulk model, using desired dimensions and parametric equations to define a helix.&#160; With this method, accurate atomistic silica nanoribbons can be generated for a range of pitch values and dimensions.&#160; The second method involves a more robust code which allows flexibility in modeling nanohelical structures.&#160; This approach utilizes a C++ code particularly written to implement pre-screening methods as well as the mathematical equations for a helix, resulting in greater precision and efficiency when creating nanospring models.&#160; Using these codes, well-defined and scalable nanoribbons and nanosprings suited for atomistic simulations can be effectively created.&#160; An added value in both open-source codes is that they can be adapted to reproduce different helical structures, independent of material.&#160; In addition, a MATLAB graphical user interface (GUI) is used to enhance learning through visualization and interaction for a general user with the atomistic helical structures.&#160; One application of these methods is the recent study of nanohelices via MD simulations for mechanical energy harvesting purposes.

Accurate modeling of nanohelical structures is important for predictive simulation studies leading to novel nanotechnology applications.&#160; Currently, software packages and codes are limited in creating atomistic helical models.&#160; We present two procedures designed to create atomistic nanohelical models for simulations, and a graphical interface to enhance research through visualization.

Spring-like materials are ubiquitous in nature and of interest in nanotechnology for energy harvesting, hydrogen storage, and biological sensing ...

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 ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

Capillary-based Centrifugal Microfluidic Device for Size-controllable Formation of Monodisperse Microdroplets

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal microfluidic device. W/O microdroplets have recently been used in powerful methods that enable miniaturized chemical experiments. Therefore, developing a versatile method to yield monodisperse W/O microdroplets is needed. We have developed a method for generating monodisperse W/O microdroplets based on a capillary-based centrifugal axisymmetric co-flowing microfluidic device. We succeeded in controlling the size of microdroplets by adjusting the capillary orifice. Our method requires equipment that is easier-to-use than with other microfluidic techniques, requires only a small volume (0.1-1 &#181;l) of sample solution for encapsulation, and enables the production of hundreds of thousands number of W/O microdroplets per second. We expect this method will assist biological studies that require precious biological samples by conserving the volume of the samples for rapid quantitative analysis biochemical and biological studies.

Here, we demonstrate a simple production method for size-controllable, monodisperse, water-in-oil (W/O) microdroplets using a capillary-based centrifugal microfluidic device. This method requires only a small sample volume and enables high-yield production. We expect this method will be useful for rapid biochemical and cellular analyses.

Here, we demonstrate a simple method for the rapid production of size-controllable, monodisperse, W/O microdroplets using a capillary-based centrifugal ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

The optical, electrical, and structural properties of dislocations and of grain boundaries in semiconductor materials can be determined by experiments performed in a scanning electron microscope. Electron microscopy has been used to investigate cathodoluminescence, electron beam induced current, and diffraction of backscattered electrons.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

Creating Sub-50 Nm Nanofluidic Junctions in PDMS Microfluidic Chip via Self-Assembly Process of Colloidal Particles

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its transparency. Due to the softness of the PDMS material, however, it is challenging to use PDMS for building nanochannels. The channels tend to collapse easily during plasma bonding. In this paper, we present an evaporation-driven self-assembly method of silica colloidal nanoparticles to create nanofluidic junctions with sub-50 nm pores between two microchannels. The pore size as well as the surface charge of the nanofluidic junction is tunable simply by changing the colloidal silica bead size and surface functionalization outside of the assembled microfluidic device in a vial before the self-assembly process. Using the self-assembly of nanoparticles with a bead size of 300 nm, 500 nm, and 900 nm, it was possible to fabricate a porous membrane with a pore size of ~45 nm, ~75 nm and ~135 nm, respectively. Under electrical potential, this nanoporous membrane initiated ion concentration polarization (ICP) acting as a cation-selective membrane to concentrate DNA by ~1,700 times within 15 min. This non-lithographic nanofabrication process opens up a new opportunity to build a tunable nanofluidic junction for the study of nanoscale transport processes of ions and molecules inside a PDMS microfluidic chip.

We propose a simple self-assembly technique of silica colloidal nanoparticles to create a nanofluidic junction between two microchannels in polydimethylsiloxane (PDMS). Using this technique, a nanoporous bead membrane with a pore size down to ~45 nm was built inside a microchannel and applied to electrokinetic preconcentration of DNA samples.

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...