TiO2 (Rutile)

Titanium dioxide crystals, chemically formulated as TiO2, belong to the tetragonal system and exhibit high refractive index, excellent thermal stability and chemical inertness, a wide optical transmittance range, high mechanical hardness, and a low defect density. They are suitable for applications in optics, electronics, and functional thin film growth. The smooth, double-sided polished surface of TiO2 substrates allows for high-quality thin film deposition, nonlinear optical devices, polarization elements, photocatalysis, and optoelectronic devices.

Artificial titanium dioxide crystals are produced by flame fusion or float zone melting. The flame fusion method uses a high-temperature flame to rapidly melt high-purity titanium dioxide powder, depositing it layer by layer along the crystal seed. By controlling the powder drop rate and the descent speed of the seed crystal platform, the molten droplets crystallize layer by layer on the seed crystal, ultimately forming a rutile titanium dioxide crystal rod.

The float zone melting method begins with a titanium dioxide rod. A ring heater is then used to create a localized molten zone within the rod. This molten zone is held in suspension by surface tension. By slowly moving the molten zone, the rod above it is melted, while the melt below recrystallizes on a seed crystal. The key to this process lies in the slow movement of the molten zone, which allows the molten titanium dioxide to transform from a liquid state into an ordered crystalline structure.

 

Physical and optical properties  

Transmission Range :	0.43 to 5.0 μm
Refractive Index :	No 2.555 at 0.69μ (1)(2)
Reflection Loss :	30% at 2μ (2 surfaces)
Absorption Coefficient :	n/a
Reststrahlen Peak :	n/a
dn/dT :	n/a
dn/dμ = 0 :	2.81μ
Density :	4.26 g/cc
Melting Point :	1840 °C
Thermal Conductivity :	12.5 (para) 8.7 (perp) W m-1 K-1
Thermal Expansion :	9.2 (para) 7.1 (perp) x 10-6 /°C
Hardness :	Knoop 879 with 500g indenter
Specific Heat Capacity :	711 J kg-1 K-1
Dielectric Constant :	160 at 1 MHz
Youngs Modulus (E) :	n/a
Shear Modulus (G) :	n/a
Bulk Modulus (K) :	n/a
Elastic Coefficients :	C11=269;C12=177;C13=146;C33=480;C44=124
Apparent Elastic Limit :	4.8 MPa (700 psi)
Poisson Ratio :	0.28
Solubility :	Insoluble in water
Molecular Weight :	79.9
Class/Structure :	Tetragonal, SnO2 rutile, P4/mmm

 

Refractive Index  

No = Ordinary Ray         Ne = Extraordinary Ray

µm	No	Ne	µm	No	Ne	µm	No	Ne
0.436	2.853	3.216	0.492	2.725	3.051	0.496	2.718	3.042
0.546	2.652	2.958	0.577	2.623	2.921	0.579	2.621	2.919
0.589	2.616	2.903	0.691	2.555	2.836	0.708	2.548	2.826
1.01	2.484	2.747	1.530	2.454	2.710	2.42	2.40	2.59
3.38	2.41	2.58	3.79	2.39	2.57	4.28	2.34	2.51
4.89	2.32	2.49	5.73	2.24	2.43

 

Working Principle of TiO2

Titanium dioxide crystals have a specific crystal structure and lattice parameters. When the lattice parameters of the growing oxide film closely match those of the titanium dioxide substrate, the titanium dioxide substrate surface acts as an atomic-level template. The atoms in the film grow according to the substrate's lattice arrangement, forming a high-quality, single-crystalline film. This lattice matching is key to achieving high-quality epitaxial growth.

At the same time, titanium dioxide is a very stable oxide. This chemical inertness ensures the purity of the film and avoids interface contamination. TiO2 also has good thermal stability and can withstand high temperatures, ensuring the physical integrity of the film growth process.

Titanium dioxide crystals have a high refractive index and a wide bandgap, making them transparent in the visible light range. This makes them ideal for fabricating optoelectronic devices, as the substrate itself does not interfere with optical signals. Furthermore, by selecting different crystal orientations, the properties of the titanium dioxide crystal surface can be manipulated, thereby affecting the growth and performance of thin films.

Production Process of TiO2 Substrates

(1) Material preparation: The purpose of this stage is to artificially produce high-quality titanium dioxide crystal blanks. The most commonly used methods are flame method and float zone melting method. The flame method process is as follows: high-purity titanium dioxide powder is sprayed into the melt area through a flame; the hydrogen-oxygen flame generates high temperature, causing the powder to melt and drip onto the seed crystal; the seed crystal slowly moves downward, and the melt gradually cools and solidifies on the seed crystal to form a single crystal. The float zone melting process is as follows: high-purity titanium dioxide raw material is pressed into a rod shape; a high-frequency induction heating coil or optical heater is used to form a narrow melting zone ("floating zone") at one end of the rod. By moving the heating coil, the melting zone slowly moves from bottom to top or from top to bottom along the rod-shaped raw material. Below the melting zone, the molten titanium dioxide will recrystallize on the seed crystal and grow into a single crystal.

(2) Preliminary processing: After obtaining the crystal blank, it must first be subjected to quality inspection, including checking for defects inside the crystal and the accuracy of the crystal orientation. After confirmation of compliance, the blank is directional cut using high-precision cutting equipment. This step is crucial because it determines the crystal orientation of the final substrate (e.g., (100), (110), (001)), which is crucial for the subsequent epitaxial growth performance of the thin film. After cutting, a preliminary substrate with the desired shape and size is obtained.

(3) Precision machining: To obtain a smooth surface, the substrate undergoes a series of precision machining operations. First, milling and fine grinding remove the roughness left by cutting and bring the substrate to a strict geometric tolerance (such as thickness, parallelism, and warpage). Next, fine polishing is performed, using abrasives of varying grit sizes to gradually grind the substrate surface until a very low roughness is achieved, typically reaching the nanometer or even sub-nanometer level.

(4) Cleaning and surface treatment: The surface of the polished substrate must be ultrasonically cleaned to completely remove residual polishing powder and dirt.

Application Areas

Titanium dioxide substrates are an important oxide substrate. Their lattice structure is well-matched with many functional oxide semiconductors (such as SrTiO3 and VO2), making them ideal for fabricating high-performance oxide semiconductor devices. High-performance memristors can be fabricated using titanium dioxide crystal substrates and oxide semiconductor thin films (such as SrTiO3 and VO2) grown on them. Memristors are resistors with memory properties, whose resistance value changes depending on the current flowing through them and the time. High-performance memristors fabricated on titanium dioxide substrates are considered key components for next-generation neuromorphic computing and non-volatile memory due to their fast switching speeds, low power consumption, and high-density integration potential.

Tags: TiO2 (Rutile)

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