Surface characterization and orientation interaction between diamond- like carbon layer structure and dimeric liquid crystals

At a Glance

Metadata	Details
Publication Date	2017-01-01
Journal	Journal of Physics Conference Series
Authors	Haritun Naradikian, M. Petrov, Boyko Katranchev, T. I. Milenov, S.S. Tinchev
Institutions	Bulgarian Academy of Sciences, Institute of Electronics
Analysis	Full AI Review Included

6CCVD Technical Analysis: Surface Engineering of Functional Carbon Layers for Liquid Crystal Alignment

Executive Summary

This documentation analyzes the research on using Diamond-Like Carbon (DLC) and Amorphous Carbon (a-C) films to control the alignment and anchoring strength of dimeric liquid crystals (LCs). The findings are highly relevant for the development of advanced optical devices and sensors requiring precision surface engineering using CVD carbon materials.

Core Achievement: Demonstrated control over Liquid Crystal (LC) alignment by tuning the $sp^2/sp^3$ hybridization ratio of thin carbon films, synthesized via PECVD.
Mechanism Verified: The orientation effect is dominated by $\pi-\pi$ electronic interaction between the carbon film and the LC molecules, strongly expressed by the $sp^2$ content.
Performance Metrics: High $sp^2$ amorphous carbon (C_02) achieved the maximum observed anchoring energy ($W_s = 0.59 \times 10^{-5}$ Jm^-2) among the tested DLC/a-C surfaces.
Material Differentiation: Films with high $sp^3$ (tetrahedral amorphous carbon, ta-C) showed significantly lower anchoring, preferring homeotropic orientation, compared to high $sp^2$ films which favored planar orientation.
Relevance to 6CCVD: This research validates the need for precise $sp^2$ content control in CVD processes. 6CCVD provides custom MPCVD substrates that can be engineered with specific surface terminations or functional carbon layers to replicate or exceed these anchoring capabilities for demanding applications.
Application Potential: Results are critical for designing robust, high-performance LC cells, optical windows, and electro-optic components leveraging the chemical and thermal stability of carbon/diamond.

Technical Specifications

The following table summarizes the key physical and performance data extracted from the experimental results, including layer compositions, physical dimensions, and anchoring metrics.

Parameter	Value	Unit	Context
LC Cell Thickness (d)	12	µm	Gap maintained by Mylar spacers
Cooling Rate (N Phase)	1.0	°C min^-1	Used for stabilizing $\pi$-inversion walls during measurement
Elastic Constant (K)	10^-11	J m^-1	Assumed for Neel type wall calculation
C_02 Layer Thickness	12-14	nm	Amorphous carbon (a-C) film
TCH_03 Layer Thickness	140	nm	Hydrogenated tetrahedral carbon (ta-C:H) film
C_02 sp²/sp³ Ratio	0.90	N/A	Highest $sp^2$ content; highest DLC/a-C planar anchoring
TC_04 sp²/sp³ Ratio	0.55	N/A	Lower $sp^2$ content; tetrahedral carbon structure
Nitrogen Doping (TCH)	0.2-0.4	at%	Doping level used to reduce layer resistance
Anchoring Energy ($W_s$) C_02/7OBA	0.59 × 10^-5	Jm^-2	Maximum DLC/a-C anchoring strength
Anchoring Energy ($W_s$) SWCNT/7OBA	16 × 10^-5	Jm^-2	Reference strongest anchoring (Carbon Nanotubes)
Extrapolation Length (L) C_02/7OBA	1.68	µm	Measure of anchoring strength
Extrapolation Length (L) TCH_03/8OBA	6.75	µm	Weakest anchoring among tested DLC/a-C films

Key Methodologies

The study utilized Plasma Enhanced Chemical Vapor Deposition (PECVD) to produce four distinct carbon film types on ITO-coated glass substrates. The structural and compositional properties were then correlated with LC alignment performance.

Substrate & Deposition:
- Thin carbon films (a-C and DLC) were deposited onto Indium-Tin Oxide (ITO) coated glass substrates.
- The films were grown using the Plasma Enhanced Chemical Vapor Deposition (PECVD) method.
- The crucial $sp^2/sp^3$ hybridization ratio was controlled by modulating the plasma voltage during deposition.
Material Design (DLC Types):
- C_02: Highly $sp^2$-dominated Amorphous Carbon (a-C).
- TC_04: $sp^3$-dominated Tetrahedral Amorphous Carbon (ta-C).
- TCH_03 & TCH_05: Hydrogenated ta-C films (ta-C:H), doped with 0.2-0.4 at% Nitrogen to decrease layer resistance.
Structural Characterization:
- Ellipsometry: Used to determine film thickness (ranging from 12 nm to 140 nm).
- X-ray Photoelectron Spectroscopy (XPS): Measured elemental composition (C, O, N content) and determined the $sp^2/sp^3$ ratio.
- Raman Spectroscopy (633 nm He-Ne laser): Confirmed phase composition by analyzing the intensity ratio and dispersion of the D (disorder) and G (graphitic) bands.
LC Measurement:
- LC material (7OBA or 8OBA) was filled into 12 µm gap cells.
- The nematic (N) phase micro-textures and $\pi$-inversion walls were monitored using an optical microscope in crossed polarizers (PLA) while cooling at 1 °C min^-1.
- Anchoring energy ($W_s$) and extrapolation length (L) were calculated using the Rapini-Papoular formula, derived from the measured wall width ($l$).

6CCVD Solutions & Capabilities

This research demonstrates that controlled surface chemistry, specifically the $sp^2/sp^3$ hybridization ratio, is paramount for engineering interfacial electronic interactions ($\pi-\pi$ bonding) essential for high-performance LC alignment. 6CCVD, as an expert in custom MPCVD diamond, offers materials and engineering services perfectly suited to advance and commercialize this type of functional carbon research.

Applicable Materials

The ideal material solution depends on whether the carbon film is required as a robust, bulk substrate or as an ultra-precise coating.

Optical Grade Single Crystal Diamond (SCD): Required for applications where high mechanical durability, extreme thermal stability, and low defect density are crucial. SCD wafers (up to 500 µm thickness) provide an atomically smooth base (Ra < 1nm) necessary for depositing highly uniform orientation layers.
Engineering Grade Polycrystalline Diamond (PCD): Ideal for large-area applications (up to 125mm) where the intrinsic grain boundaries can be leveraged or controlled. PCD substrates offer robust thermal performance far exceeding the glass/ITO used in the study.
Custom BDD Layers: For applications requiring controlled surface conductivity or electrochemical activity, 6CCVD can integrate a Boron-Doped Diamond (BDD) layer. The resulting highly conductive surface can enhance the “double electric layer” phenomenon mentioned in the paper, potentially boosting anchoring strength beyond the SWCNT reference values.

Customization Potential

Replicating and extending this research requires precise control over layer thickness, surface roughness, and contact pads. 6CCVD’s in-house capabilities meet or exceed these requirements:

Research Requirement	6CCVD Custom Capability	Application Benefit
Film Thickness	SCD/PCD layers from 0.1 µm up to 500 µm. Custom seed layer tuning.	Provides highly reproducible diamond bases for functional top coatings.
Surface Finish	SCD surfaces polished to Ra < 1nm. PCD surfaces polished to Ra < 5nm (inch-size).	Ensures minimal scattering loss and controlled interfaces critical for optical device performance.
Interface Layers	Capability to deposit and control the $sp^2/sp^3$ ratio in DLC/carbon interface layers.	Allows researchers to precisely tune $\pi-\pi$ electronic bonding strength for optimal planar or homeotropic LC orientation.
Electrode Fabrication	Internal metalization services (Au, Pt, Pd, Ti, W, Cu).	Enables integration of high-conductivity, custom electrode patterns necessary for manipulating the LC double electric layer effect.
Custom Dimensions	Plates/wafers up to 125mm (PCD). Precision laser cutting services.	Supports transition from lab-scale coupons (like those studied) to industrial-scale production prototypes.

Engineering Support

The successful implementation of functional carbon interfaces for liquid crystal alignment is a complex challenge spanning material science and electro-optics. 6CCVD’s in-house PhD engineering team specializes in tailoring MPCVD diamond properties to meet specific research and application goals. We can assist with:

Material Selection: Guidance on selecting the optimal diamond material (SCD vs. PCD vs. BDD) based on required thermal, electrical, and optical constraints.
Surface Termination: Customizing surface functionalization and interface layers to control the hydrogen bonding and $\pi-\pi$ interactions crucial for advanced LC orientation projects.
Design & Manufacturing: Support in developing custom dimensions, metalization layouts, and thickness tolerances for prototype development.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available) to accelerate your research timeline.

View Original Abstract

Diamond-like carbon (DLC) and amorphous carbon films are very promising type of semiconductor materials. Depending on the hybridization sp2/sp3 ratio, the material’s band gap varies between 0.8 and 3 eV. Moreover carbon films possess different interesting for practice properties: comparable to the Silicon, Diamond like structure has 22-time better thermal conductivity etc. Here we present one type of implementation of such type nanostructure. That is one attempt for orientation of dimeric LC by using of pre-deposited DLC layer with different ratio of sp2/sp3 hybridized carbon content. It could be expected a pronounced π1-π2interaction between s and p orbital levels on the surface and the dimeric ring of LC. We present comparison of surface anchoring strengths of both orientation inter-surfaces DLC/dimeric LC and single wall carbon nanotubes (SWCNT)/dimeric LC. The mechanism of interaction of dimeric LC and activated surfaces with DLC or SWCNT will be discussed. In both cases we have π-π interaction, which in combination with hydrogen bonding, typical for the dimeric LCs, influence the LC alignment. The Raman spectroscopy data evidenced the presence of charge transfer between contacting hexagonal rings of DLC and the C = O groups of the LC molecules.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1742-6596/780/1/012010