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6CCVD Research

Rhombic Coulomb diamonds in a single-electron transistor based on an Au nanoparticle chemically anchored at both ends

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2016-01-01
JournalNanoscale
AuthorsYasuo Azuma, Yuto Onuma, Masanori Sakamoto, Toshiharu Teranishi, Yutaka Majima
InstitutionsKyoto Bunkyo University, Kyoto University Institute for Chemical Research
Citations17
AnalysisFull AI Review Included

6CCVD Technical Analysis & Documentation

Section titled “6CCVD Technical Analysis & Documentation”

Executive Summary

Section titled “Executive Summary”

This paper demonstrates the fabrication and characterization of a highly symmetric, chemically anchored Single-Electron Transistor (SET) utilizing an Au nanoparticle (NP) core. The key findings directly validate methods crucial for next-generation molecular and quantum electronic devices, creating specific requirements addressable by 6CCVD’s advanced diamond materials.

  • Symmetric SET Achievement: Clear, reproducible rhombic Coulomb diamonds were observed, confirming near-perfect symmetry in the double-barrier tunneling junctions (DBTJs).
  • Low and Balanced Resistance: Theoretical fitting confirmed highly comparable and ultra-low tunneling resistances ($R_{1} = 4.5\ \text{M}\Omega$, $R_{2} = 4.8\ \text{M}\Omega$) and capacitances ($C_{1} \approx C_{2}$), a prerequisite for symmetric SET performance.
  • Improved Chemical Anchoring: The use of two-methylene-group shorter octanedithiol (C8S2) anchor molecules successfully reduced the tunneling resistance by more than an order of magnitude compared to previous long-chain chemistries.
  • Precise Nanoscale Fabrication: The device fabrication relied on high-fidelity Electron Beam Lithography (EBL) combined with self-terminated electroless Au plating to achieve sub-20 nm nanogap electrodes.
  • Cryogenic Operation: Electrical measurements were successfully conducted at an ultra-low temperature of $9\ \text{K}$, confirming device stability under extreme conditions.
  • Molecular Electronics Validation: This research validates precise chemical assembly techniques for forming functional, subnanometer-scale electronic architectures, positioning the technology for molecular computing and high-sensitivity charge detection.

Technical Specifications

Section titled “Technical Specifications”

Hard data extracted from the experimental measurements and orthodox model fitting.

ParameterValueUnitContext
Nanoparticle Core Diameter6.2 ± 0.8nmAu NP core (Coulomb Island)
Operating Temperature (T)9KCryogenic measurement requirement
Source Tunneling Resistance (R1)4.5MΩEvaluated via orthodox model fitting
Drain Tunneling Resistance (R2)4.8MΩEvaluated via orthodox model fitting (R1/R2 ≈ 1)
Tunneling Capacitance Ratio (C1/C2)1.4/1.3aF/aFIndicative of SET symmetry
Gate 1 Capacitance (CG1)0.025aFExtracted from Coulomb diamond slopes
Gate 2 Capacitance (CG2)0.015aFExtracted from Coulomb diamond slopes
Applied Drain Voltage (VD)14 - 50mVRange for $I_{D}$-$V_{G1}$ measurements
Gate Dielectric Thickness50nmSiO2 layer on Si substrate

Key Methodologies

Section titled “Key Methodologies”

The SET fabrication utilized a hybrid top-down (lithography) and bottom-up (chemical assembly) approach, focusing on creating precise interfaces and stable molecular anchors.

  1. Electrode Patterning (Top-Down):

    • Initial electrodes (Source, Drain, Gate 1, Gate 2) were defined by Electron Beam Lithography (EBL) and lift-off.
    • Electrode stack consisted of 2 nm Titanium (Ti) adhesion layer and 10 nm Gold (Au) layer.
    • Substrate: $50\ \text{nm}\ \text{SiO}_{2}$ on a Si wafer.

  2. Nanogap Refinement:

    • Initial Au electrodes were cleaned using O2 plasma.
    • Electroless Au plating in an Au iodine solution was used to reduce the size of the initial nanogap electrodes through a self-termination reaction, achieving the required sub-20 nm gap width.

  3. Self-Assembled Monolayer (SAM) Formation:

    • C6S Preparation: Electrodes were immersed in 1 mM Hexanethiol [$\text{C}_{6}\text{S}$] solution in ethanol (12 hours) to form the base SAM.
    • C8S2 Integration: The sample was then immersed in 1 mM Octanedithiol [$\text{C}{8}\text{S}{2}$] solution in ethanol (12 hours) to yield a $\text{C}{8}\text{S}{2}/\text{C}_{6}\text{S}$ mixed SAM. C8S2 acted as the short-chain anchor molecule.

  4. Nanoparticle Chemical Anchoring:

    • $\text{C}10\text{S}$-protected Au nanoparticles (core diameter $6.2 \pm 0.8\ \text{nm}$) were dispersed in toluene.
    • The sample was immersed in the NP solution (12 hours). The C8S2 anchor molecules chemically bonded the NP core to both the Source and Drain electrodes, establishing the double-barrier tunneling junction (DBTJ).

  5. Electrical Testing:

    • Measurements ($\text{I}{D}$-$\text{V}{D}$ and $\text{I}{D}$-$\text{V}{G}$) were performed in a vacuum chamber ($\approx 10^{-5}\ \text{Pa}$) at $9\ \text{K}$ using a semiconductor parameter analyzer.

6CCVD Solutions & Capabilities

Section titled “6CCVD Solutions & Capabilities”

This research highlights the critical need for ultra-high quality, atomically smooth substrates and precise metalization capabilities—areas where 6CCVD’s MPCVD diamond excels. By moving beyond traditional $\text{SiO}_{2}/\text{Si}$ platforms, researchers can leverage the superior thermal, mechanical, and electronic properties of diamond for robust quantum and molecular devices.

Applicable Materials for Replication and Extension

Section titled “Applicable Materials for Replication and Extension”
Research Requirement / Goal6CCVD Applicable MaterialKey Advantage
High Stability & Cryo-Compatibility (Alternative Substrate)Optical Grade Single Crystal Diamond (SCD)Exceptional thermal conductivity stabilizes devices at 9 K; atomically smooth surface (Ra < 1 nm) is ideal for EBL and molecular assembly.
Intrinsic Diamond Electrodes (Beyond Metal Contacts)Heavy Boron-Doped Diamond (BDD)Low-resistivity BDD films serve as robust, chemically stable electrodes, allowing integration of the SET architecture directly onto the functional diamond surface.
Large-Area Scalability (PCD for Manufacturing/Arrays)Polycrystalline Diamond (PCD) WafersCost-effective synthesis of inch-size PCD wafers (up to 125 mm) with smooth surfaces (Ra < 5 nm) for scalable SET array fabrication.

Customization Potential & Engineering Support

Section titled “Customization Potential & Engineering Support”

6CCVD offers specialized services critical to replicating and advancing SET and molecular device research:

  • Custom Dimensions and Shaping: While the experiment used small chips, 6CCVD provides custom diamond plates and wafers up to 125 mm (PCD). We offer precision laser cutting for substrates tailored to unique cryogenic probe stages or specific chip sizes required by wafer-scale integration processes.
  • Precision Polishing: Achieving high-fidelity nanogaps via EBL demands extremely flat starting surfaces. 6CCVD guarantees ultra-low roughness polishing ($\text{Ra} < 1\ \text{nm}$ for SCD), ensuring optimal lithographic transfer and minimizing geometric asymmetry that affects $R_{1}/R_{2}$ ratios.
  • Integrated Metalization Services: The paper required Ti/Au stack electrodes. 6CCVD has internal capability for depositing complex multi-layer metalization schemes directly onto SCD or PCD surfaces, including Au, Pt, Pd, Ti, W, and Cu, ensuring robust, low-resistance electrical contacts for SET operation.
  • Thickness Control: We provide precise control over film thickness, offering SCD and PCD layers from $0.1\ \text{”m}$ up to $500\ \text{”m}$, allowing researchers to optimize the diamond platform’s electronic isolation and thermal sinking properties.
  • Expert Engineering Support: 6CCVD’s in-house PhD team can assist with material selection for similar Molecular Single-Electron Transistor (MSET) and quantum device projects, advising on optimal doping levels (for BDD electrodes) and surface preparations for novel chemical anchoring techniques.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Rhombic Coulomb diamonds are clearly observed in a chemically anchored Au nanoparticle single-electron transistor. The stability diagrams show stable Coulomb blockade phenomena and agree with the theoretical curve calculated using the orthodox model. The resistances and capacitances of the double-barrier tunneling junctions between the source electrode and the Au core (R1 and C1, respectively), and those between the Au core and the drain electrode (R2 and C2, respectively), are evaluated as 4.5 MΩ, 1.4 aF, 4.8 MΩ, and 1.3 aF, respectively. This is determined by fitting the theoretical curve against the experimental Coulomb staircases. Two-methylene-group short octanedithiols (C8S2) in a C8S2/hexanethiol (C6S) mixed self-assembled monolayer is concluded to chemically anchor the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes even when the Au nanoparticle is protected by decanethiol (C10S). This is because the R1 value is identical to that of R2 and corresponds to the tunneling resistances of the octanedithiol chemically bonded with the Au core and the Au electrodes. The dependence of the Coulomb diamond shapes on the tunneling resistance ratio (R1/R2) is also discussed, especially in the case of the rhombic Coulomb diamonds. Rhombic Coulomb diamonds result from chemical anchoring of the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2009 - Bottom-up Nanofabrication, Volume 3 Self-Assemblies-I

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