Name: a modular microfluidic technology for systematic studies of colloidal semiconductor nanocrystals
Uploaded: 2018-05-10T15:00:00
Description: Watch this Scientific Journal Video about A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals at JoVE.com

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 t...

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W., Yu, Y., Dou, L., Yang, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solution-phase+synthesis+of+cesium+lead+halide+perovskite+nanowires.">Solution-phase synthesis of cesium lead halide perovskite nanowires.</a> Journal of the American Chemical Society. 137 (29), 9230-9233 (2015).</li><li>Lignos, I., Stavrakis, S., Nedelcu, G., Protesescu, L., deMello, A. J., Kovalenko, M. 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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.

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	<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 ...

The overall purpose of this procedure is to assemble and use a microfluidic high-throughput screening platform for systematic inline studies of the reaction pathways of colloidal semiconductor nanocrystals. This platform provides researchers with access to a full absorption and emission spectra within a parameter space that was previously inaccessible. Beyond the extended parameter range, the high sampling rate and low chemical consumption allows many more conditions to be tested at a fraction of the cost compared to flask-based screening.Further implementation of this system will improve the pace of research and, therefore, bring us closer to commercial scale production of low-cost, high-efficiency quantum dot based photovoltaic cells. To begin assembling the microfluidic platform, fix a linear translation stage lengthwise on an aluminum optical breadboard. Fix four optical post holders on the board around the track and post two holders on the stage platform.Connect an optical post to each of the four corners of the junction stage then place the optical posts into the four post holders, mount. Connect the flow cell to the optical posts on the translation stage platform. Then, cut a length of FEP tubing as the reactor line and three lengths of ETFE tubing as the precursor feed lines.Fit each line with a flangeless ferrule and nut at one end. Fit the other end to the precursor lines with gas-tight syringe fittings and flow valves as needed for the syringe configuration to be used. Connect the reactor and precursor feed lines to a custom-built, four-way cross junction so that the reactor line will be next to the flow cell.Place the cross junction in the junction mounting stage. Feed the precursor lines through the channels of the junction stage. Then, thread the reactor line through a sampling port.Fit the sampling port through the flow cell being careful not to stretch or crimp the reactor line as the sampling port is moved along the line. Connect the port to the junction stage. Fasten the precursor line cover onto the junction stage to secure the tubing and sampling port in place.Connect the desired number of sampling ports and extension units to the assembly keeping the modules as straight and level as possible to avoid distorting or damaging the tubing. Connect a support bracket to the outlet of the last sampling port so that the bracket is under the reactor tubing outlet. Secure the support bracket on the remaining two optical posts.Guided by a carpenter's level, adjust the outlet support structure until the reactor assembly is straight and level. Then, use fiber optic patch cords to connect a spectrometer and LED in a deuterium halogen light source to the flow cell ports. Test the translation stage to ensure that the cables do not restrict the flow cell movement.To begin preparing the precursors, combine 109 milligrams of tetraoctylammonium bromide, one milliliter of oleic acid and 14 milliliters of toluene in a 20-milliliter vial equipped with a stir bar. Seal the vial and stir the mixture vigorously at room temperature until clear and colorless to form the bromide precursor solution. Next, place 0.6 millimoles of cesium hydroxide, 0.6 millimoles of lead two oxide and three milliliters of oleic acid in an eight-milliliter vial equipped with a stir bar.Seal the vial with a septum cap. Pierce the septum with a needle as a vent. Vigorously stir the mixture at 160 degrees Celsius until clear and colorless.Then, with the vent needle still in place, heat the mixture in an oven at 120 degrees Celsius for one hour. After that, remove the venting needle and allow the cesium lead mixture to cool to room temperature in open air. Combine 0.5 milliliters of the concentrated cesium lead mixture with 47.5 milliliters of toluene in a sealed 50-milliliter vial equipped with a stir bar.Vigorously stir the mixture until homogenous to obtain the dilute cesium lead precursor solution. Load the bromide and cesium lead precursors into 25-milliliter glass syringes. Fill an eight-milliliter stainless steel syringe with nitrogen gas from a gas cylinder.Connect the liquid precursor syringes and the nitrogen gas syringe to the precursor lines. If absorption reference spectra will be collected using a blank solution, connect a syringe filled with the blank solution to one of the liquid feed lines. Mount the syringes on computer controlled syringe pumps then thread the reactor line through the septum of a 50-milliliter vial.Pressurize the vial with nitrogen gas via a two-stage gas regulator to complete the setup. Once ready to begin the experiment, open the automated operation software and set the path to the folder in which the data should be saved. Select the USB connection address for the spectrometer.Set the integration time, the number of spectra to average and the number of spectra to save for both absorption and fluorescence. If multiphase flow will be characterized, click the multiphase button, set the minimum sample length so that approximately two complete gas liquid oscillations will pass the sampling point. Set the number of samples to be taken within that window.Then, set the communication addresses for the syringe pumps and fill in the syringe inner diameters for the syringes in use. Leave the diameters of the extraneous syringes at the default values. If absorption reference spectra are to be collected, set the flow rate of the syringe containing the reference solution, or precursor, to 300 microliters per minute.Then, either select a previously optimized set of stage locations or choose an appropriate reference file and a stage position window size. Ensure that the stage increment is 0.05 millimeters and the startup passes value is eight. Fill in the volume in microliters of the reactor tubing from the center of the junction to the final sampling port as the system volume.Ensure that the minimum equilibration time is set to 10 seconds. Double check all values and then click run. Set up to 30 flow rate configurations to test leaving unused syringe inputs blank.Choose whether to save reference spectra if applicable. The system will run through the selected conditions and will automatically shut down when finished. A series of fluorescence and absorbance spectra were collected in a single pass of a multiphase cesium lead bromide perovskite nanocrystal system with an average slug velocity of approximately 0.2 centimeters per second.Similar sets of spectra were collected at other flow rates and reactor lengths. Plotting the peak fluorescence wavelength as a function of residence time revealed the trend of higher peak fluorescent wavelengths at lower fluid velocities. A notable difference in peak fluorescence wavelength was observed when the slug velocity was increased from 75 millimeters per second to 130 millimeters per second while retaining a residence time of 0.9 seconds.Once assembled, this system has the capability to collect up to 30, 000 unique optical spectra within a single day all within a mass transfer controlled sampling space. By applying this platform to other colloidal semiconductor synthesis, researchers will gain access to a broad range of nanocrystal growth information with far greater accuracy and precision than through conventional flask-based strategies at a fraction of the cost and time.

a-modular-microfluidic-technology-for-systematic-studies-colloidal

Research

JoVE Journal

Engineering

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 ...