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	Research Organization of Information and Systems, National Institute for Materials Science
Citations	27
Analysis	Full AI Review Included

Core Value Proposition: A novel, quantitative method using hydrogenated diamond (H-diamond) Electric Double Layer Transistors (EDLTs) to characterize the Electric Double Layer (EDL) effect and its suppression in inorganic solid electrolytes.
High EDL Performance: The LSZO (Li-Si-Zr-O) solid electrolyte EDLT achieved massive EDL-induced hole density modulation (up to three orders of magnitude) and a high EDL capacitance (up to 14 µF/cm²), comparable to liquid electrolytes.
Mechanism of Suppression: The EDL effect was strongly suppressed (capacitance reduced by three orders of magnitude) when using the LLTO (La-Li-Ti-O) electrolyte, which contains redox-active Ti ions.
Redox Neutralization Confirmed: In situ STEM-EELS confirmed that the suppression mechanism is internal charge neutralization within the LLTO layer, driven by the redox reaction of Ti ions (Ti³⁺ to Ti⁴⁺), rather than interruption of Li⁺ transport.
EDL Structure Validation: In situ Hard X-ray Photoelectron Spectroscopy (HAXPES) confirmed the formation of a classic 2-layer EDL structure at the Au/LSZO interface, featuring a steep potential drop (several MV/cm) in a sub-nm Helmholtz layer and a gentler slope in a several-nm diffusion layer.
Engineering Impact: The technique allows for quantitative evaluation of EDL charging mechanisms, crucial for optimizing solid electrolyte interfaces in next-generation solid-state ionic devices.

Parameter	Value	Unit	Context
Hole Density Modulation (LSZO)	3	orders of magnitude	H-diamond channel, 0 to -1 V V_G
Maximum Hole Density (LSZO)	2.7 x 10¹³	cm^-2	H-diamond/LSZO interface
EDL Capacitance (LSZO)	Up to 14	µF/cm²	H-diamond/LSZO interface
EDL Capacitance (LLTO/LSZO)	8.7	nF/cm²	H-diamond/LLTO/LSZO interface (Suppressed)
Ionic Conductivity (LSZO)	5.7 x 10^-9	S/cm	At Room Temperature (RT)
Ionic Conductivity (LLTO)	8.9 x 10^-9	S/cm	At Room Temperature (RT)
Activation Energy (Ionic)	0.654 (LSZO), 0.687 (LLTO)	eV	Ionic conduction
Activation Energy (Response)	0.657	eV	Switching response (1/τ)
Switching Response Time	< 1	ms	LSZO device, at 340 K
Potential Drop Magnitude	Several	MV/cm	Steep drop in sub-nm Helmholtz layer (HAXPES)
Helmholtz Layer Thickness	0.13 to 0.7	nm	Au/LSZO interface (Simulated)
Diffusion Layer Thickness	10 to 20	nm	Au/LSZO interface (Simulated)
Electrolyte Film Thickness	700	nm	LSZO or LLTO
LLTO Interlayer Thickness	5	nm	LLTO/LSZO device

H-Diamond Channel Fabrication:
- H-diamond homoepitaxial film (500 nm thick) was deposited on Ib-type High-Pressure High-Temperature (HPHT) diamond (100) single crystal.
- Deposition utilized Microwave Plasma Chemical Vapor Deposition (MPCVD) at 1213 K.
- Gas fluxes were fixed at H₂ (1000 sccm) and CH₄ (0.5 sccm).
Electrode and Contact Deposition:
- Pd, Ti, and Au thin films (10, 10, 200 nm thick, respectively) were deposited by electron beam evaporation.
- Pd insertion was used to ensure good ohmic contact to the H-diamond.
Solid Electrolyte Deposition:
- LSZO (Li-Si-Zr-O) and LLTO (La-Li-Ti-O) films (700 nm thick) were deposited using Pulsed Laser Deposition (PLD).
- A 193-nm ArF excimer laser was used for efficient ablation of the wide bandgap materials.
Electrical and Transport Characterization:
- Electrical characteristics were measured using a semiconductor parameter analyzer and Hall measurement setup (delta mode, ±200 nA current pulses).
- Ionic conductivities and device response were measured using AC impedance spectroscopy (1 MHz to 0.1 Hz).
In Situ Interface Analysis:
- STEM-EELS: Used to investigate EDL suppression mechanism by observing Li-K and Ti-L edges near the diamond/LLTO interface under applied DC voltage (0 V and 1 V).
- HAXPES: Used to assess the potential drop profile at the Au electrode/LSZO interface under applied DC voltage (0 V to 1 V), confirming the Helmholtz and diffusion layer structure.

All-Solid-State Lithium Ion Batteries (ASS-LIBs): The method provides a critical tool for evaluating and optimizing the interfacial resistance and charge transfer kinetics at the solid electrolyte/electrode interface, a major bottleneck in ASS-LIB performance.
Memristors and Atomic Switches: Relevant for characterizing the EDL effect in solid-state ionic thin films used in Electrochemical Metallization (ECM) devices, which rely on precise ion migration and charge accumulation for switching.
Neuromorphic Devices: Applicable to solid-state ionic devices designed for analog computation and neural network functions, where EDL dynamics control switching properties and plasticity.
Advanced Nanoelectronics (ICT): Used for developing high-performance EDLTs for physical property tuning (e.g., magnetization, bandgap) and high-density charge accumulation.
Sensor Technology: Relevant for developing highly sensitive ion sensors based on electrolyte-gated FETs, leveraging the large capacitance achieved by the EDL effect.