The electric double layer effect and its strong suppression at Li+ solid electrolyte/hydrogenated diamond interfaces

At a Glance

Metadata	Details
Publication Date	2021-08-06
Journal	Communications Chemistry
Authors	Takashi Tsuchiya, Makoto Takayanagi, Kazutaka Mitsuishi, Masataka Imura, Shigenori Ueda
Institutions	Tokyo University of Science, National Institute for Materials Science
Citations	27
Analysis	Full AI Review Included

Technical Documentation: Advanced Diamond Interfaces for Solid-State Ionics

Source Paper Analysis: The electric double layer effect and its strong suppression at Li+ solid electrolyte/hydrogenated diamond interfaces (Communications Chemistry, 2021)

Executive Summary

This research successfully demonstrates the quantitative characterization of the Electric Double Layer (EDL) effect at solid electrolyte/electrode interfaces using hydrogenated diamond (H-diamond) based EDL Transistors (EDLTs). The findings are critical for the development of next-generation all-solid-state lithium-ion batteries (ASS-LIBs) and nanoelectronic devices.

Core Achievement: H-diamond (100) homoepitaxial film was utilized as a chemically stable, ion-blocking channel to isolate and measure electrostatic carrier doping induced solely by the EDL effect.
Performance Metrics: The Li-Si-Zr-O (LSZO) electrolyte device achieved hole density modulation up to three orders of magnitude (2.7 x 10¹³ cm^-2) and exhibited a massive EDL capacitance up to 14 µF/cm², comparable to liquid electrolytes.
Material Science Insight: The study confirmed that the EDL effect is strongly suppressed (capacitance reduced by three orders of magnitude) when using Li-La-Ti-O (LLTO) electrolyte, attributed to redox activity (Ti³⁺ to Ti⁴⁺) neutralizing the EDL charge.
Interface Characterization: In situ Hard X-ray Photoelectron Spectroscopy (HAXPES) confirmed the formation of a steep potential drop, characteristic of a sub-nm thick Helmholtz layer and a several-nm thick diffusion layer, even in rigid inorganic solid electrolytes.
Mechanism Validation: The switching response speed of the EDLT was shown to be quantitatively governed by the ionic conductivity of the LSZO electrolyte, validating the standard EDL charging mechanism.
6CCVD Relevance: The H-diamond films used were grown via MPCVD, directly aligning with 6CCVD’s core expertise in high-quality, custom SCD and BDD materials required for replicating and scaling these advanced ionic devices.

Technical Specifications

The following hard data points were extracted from the analysis of the H-diamond EDLT devices and electrolyte materials:

Parameter	Value	Unit	Context
Diamond Film Type	Homoepitaxial H-diamond	N/A	(100) oriented, used as ion-blocking channel.
Diamond Film Thickness	500	nm	Deposited via MPCVD.
Electrolyte Thickness (LSZO/LLTO)	700	nm	Deposited via Pulsed Laser Deposition (PLD).
LLTO Interlayer Thickness	5	nm	Used in LLTO/LSZO device to study suppression.
LSZO Ionic Conductivity (RT)	5.7 x 10^-9	S/cm	Measured at Room Temperature (RT).
LLTO Ionic Conductivity (RT)	8.9 x 10^-9	S/cm	Measured at Room Temperature (RT).
LSZO Activation Energy (E_a)	0.654	eV	Derived from Arrhenius plot.
Maximum Hole Density (LSZO)	2.7 x 10¹³	cm^-2	Achieved at V_G = -1 V (3 orders of magnitude modulation).
Hole Mobility (LSZO Device)	150 to 50	cm²/Vs	Steep decrease observed due to enhanced phonon scattering.
EDL Capacitance (LSZO)	Up to 14	µF/cm²	Calculated from hole density variation.
Helmholtz Layer Thickness	< 1	nm	Determined by HAXPES simulation at Au/LSZO interface.
Diffusion Layer Thickness	~20	nm	Determined by HAXPES simulation at Au/LSZO interface.
Switching Response Time (340 K)	< 1	ms	LSZO device ON to OFF state switching.

Key Methodologies

The experimental success hinges on precise material synthesis and advanced in situ characterization techniques, all relevant to 6CCVD’s expertise in MPCVD diamond fabrication.

H-Diamond Homoepitaxial Growth (MPCVD):
- Substrate: Ib-type High-Pressure High-Temperature (HPHT) diamond (100) single crystal.
- Method: Microwave Plasma Chemical Vapor Deposition (MPCVD).
- Temperature: 1213 K (940 °C).
- Gas Flow: H₂ (1000 sccm) and CH₄ (0.5 sccm).
- Result: 500-nm thick H-diamond film with a (100)-oriented surface, crucial for the 2DHG (Two-Dimensional Hole Gas) channel.
Electrolyte Thin Film Deposition (PLD):
- Method: Pulsed Laser Deposition (PLD) using a 193-nm ArF excimer laser.
- Materials: Li-Si-Zr-O (LSZO) and Li-La-Ti-O (LLTO).
- Thickness: 700 nm (Electrolyte), 5 nm (LLTO interlayer).
Electrode Fabrication and Metalization:
- Ohmic Contacts: Pd (10 nm) was inserted to achieve good ohmic contact, followed by Ti (10 nm) and Au (200 nm) deposition via electron beam evaporation.
- Gate Electrode: LiCoO₂ (LCO) and Pt thin films.
- Conductive Diamond: Boron-Doped Diamond (BDD) was used as the working electrode for in situ STEM-EELS experiments to ensure sufficient electrical conductance.
In Situ Characterization:
- Hall Measurement: Used to determine hole density and mobility modulation under varying gate voltage (V_G).
- In Situ Hard X-ray Photoelectron Spectroscopy (HAXPES): Non-destructive method used to assess the potential drop profile (Helmholtz and diffusion layers) at the Au/LSZO interface.
- In Situ Scanning Transmission Electron Microscope - Electron Energy Loss Spectroscopy (STEM-EELS): Used to investigate the EDL suppression mechanism by observing Li-K and Ti-L edges, confirming Ti ion redox activity (Ti³⁺ to Ti⁴⁺).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-purity, custom diamond materials necessary to replicate, scale, and advance this research into commercial solid-state ionic devices. Our MPCVD expertise ensures precise control over material properties, orientation, and doping required for high-performance EDLTs and ASS-LIB interfaces.

Applicable Materials

To replicate or extend the H-diamond EDLT research, 6CCVD recommends the following materials from our catalog:

Material Specification	6CCVD Product Line	Application in Research	6CCVD Capability Match
Single Crystal Diamond (SCD)	Electronic Grade SCD (100)	Ion-blocking channel, 2DHG formation.	Custom SCD wafers up to 10mm thickness, precise (100) orientation control.
Thin Film SCD	SCD Homoepitaxial Film	Active channel layer (500 nm thickness used).	SCD thickness control from 0.1 µm to 500 µm, ensuring high crystalline quality (Ra < 1 nm polishing available).
Boron-Doped Diamond (BDD)	Heavy Boron Doped SCD/PCD	Conductive electrode for in situ EELS/HAXPES experiments.	Custom BDD doping levels for specific conductivity requirements (e.g., highly conductive electrodes).

Customization Potential

The success of this research relies on precise interface engineering and specific electrode geometries. 6CCVD offers comprehensive customization services to meet these demands:

Custom Dimensions: While the paper used small hall-bar geometries, 6CCVD can supply SCD or PCD plates/wafers up to 125mm in diameter, enabling large-scale device fabrication and integration.
Precise Thickness Control: We guarantee SCD film thickness control from 0.1 µm up to 500 µm, allowing researchers to optimize the active channel layer depth for maximum EDL effect.
Advanced Metalization Services: The paper utilized Pd/Ti/Au contacts. 6CCVD provides in-house, high-precision deposition of critical metals including Au, Pt, Pd, Ti, W, and Cu, ensuring robust ohmic contacts and gate electrodes tailored to specific electrochemical requirements.
Surface Preparation: For optimal H-termination and 2DHG formation, 6CCVD offers ultra-smooth polishing (Ra < 1 nm for SCD) and precise surface termination services.

Engineering Support

The complex interplay between ionic conductivity, redox activity (as seen with Ti ions in LLTO), and EDL capacitance requires deep material expertise. 6CCVD’s in-house PhD team specializes in diamond electrochemistry and solid-state interfaces. We can assist engineers and scientists with:

Material Selection: Optimizing SCD or BDD specifications (doping, orientation, surface termination) for similar All-Solid-State Battery (ASS-LIB) or Memristor projects.
Interface Optimization: Consulting on metalization stacks and surface preparation techniques to ensure stable, high-performance ion-blocking or mixed-conducting interfaces.
Recipe Replication: Providing MPCVD diamond films grown under specific conditions to replicate the high-quality H-diamond homoepitaxial films used in this foundational research.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s42004-021-00554-7