Aligned Growth of ZnO Nanorods
Justin Hwa
Advisors: Dr. ZL Wang, Jinhui Song
BackgroundIn recent years, ZnO nanostructures have attracted a lot of researchinterests due to their unique structure- and size-dependent electrical, optical, andmechanical properties and great potentials in a wide range of applications. Our
research focuses on the fabrication of vertically aligned and uniform ZnOnanorods, which is considered effective for the assembly of nanodevices andapplications in light emitting and field emissions. We employ the vapor-liquidsolid (VLS) process for the growth of the nanostructures because of its simple
and efficient method. By varying the conditions – temperature, pressure,
substrate position, and reactant gas partial pressure – in synthesis, different
results can be obtained. The objective of our research is to determine the best
condition(s) for growing perfectly aligned and uniform ZnO nanorods and provide
a foundation for large-scale, controlled synthesis of ZnO nanorods to be used inpractical applications.
Synthesis
Various substrates are used in our experiments, including sapphire, GaN,
and AlGaN/AlN. These substrates have ordered surfaces that allow for thealigned and upward growth of the nanorods. For the VLS process, a layer of gold,
used as a catalyst that initiates the growth, is plated on the substrates. Thesource material is prepared by mixing equal amounts of ZnO powder and carbon

powder, which catalyzes the vaporization and reduce the vaporization
temperature from ~1300°C to ~800°C. Below is the reaction process:~800°C

ZnO(s) + C(s) Æ Zn(v) + CO(v)Synthesis is conducted in a tube furnace (Figure 1) where the source material,
loaded on a boat, is placed in the middle of the alumina tube while the substrateis placed somewhere downstream as shown in Figure 2.

Figure 1 - Barnstead International Low Temperature Tube Furnace

Figure 2 – Tube furnace setup

After the source material and substrate are loaded into the tube, the ends aresealed with steel caps, which are passed through with cooling water. This
creates a temperature gradient inside the tube as the furnace heats. The caps
also provide the carrying (Ar) and reactant (O2) gases that flow from one end of
the tube to another. When all setups are finished, the furnace is heated to acertain temperature where the source material will begin to vaporize. The vapors
are carried by the flowing gas to a lower temperature region where it is depositedon the substrate. The synthesis will continue until the furnace is stopped or when
the source material is exhausted.
In past experiments, it is observed that chamber pressure is the mostdetermining factor of the result quality. The pressure controls the growth byaffecting the saturation of the vapor in the carrying gas and the reaction with theO2 reactant gas. The pressure fluctuates during synthesis as temperaturechanges and when the partial pressure of the vaporizing ZnO begins tocontribute to the overall pressure. One of my main responsibilities in the researchis to constantly adjust the pressure to a certain point, depending on theexperiment, to ensure the best precision.
Growth Mechanisms
The vapor-liquid-solid (VLS) growth process is determined by theintroduction of a metal catalyst. Gold is used as the catalyst in our experiments.
Figure 3 demonstrates the VLS process:Figure 3 – VLS growth process.

[Left]

A thin layer of gold is plated on the substrate.

[Middle] As temperature heats up, the gold forms into nanosized droplets,and begins to absorb the incoming Zn vapors.
[Right]ZnO ultimately supersaturates in the droplet as vapors arecontinued to be absorbed. When this happens, ZnO is precipitatedout at the solid-liquid interface and forms wire- or belt-likestructures.

Results and Discussion
Each experiment is recorded in a log and all parameters are written down.
Below is a sample entry:
ZnO – B46
7-22-2005
Source Materials: ZnO - 0.6g
C - 0.6g
Substrate: AlGaN:Mg catalyst, sputter 30” Au
25cm from the end
Low temperature furnace:
Small valve: 0.1 mbar
Big valve:
4.0 x 10^-2 mbar
Heat rate:
50°C/min
Keep at 950°C for 30 minutes
Gas:
Ar – 49.0 sccm
O2 – 1.0 sccm
Keep pressure at 30 mbar

Products from the experiments are checked with a scanning electron microscope(SEM) (Figure 4).

Figure 4 – SEM

Our experiment data is unlike others in that it cannot really be expressed incharts and graphs. Rather, the quality of the products we produce aredetermined visually and images are taken at different regions of the samples forrecords. Figure 4 is an example result from an experiment:

Figure 5

In Figure 5, it can be seen that a very high percentage of the nanorods arealigned and perpendicular to the surface. In addition, their dimensions are verymuch identical. Although several nanorods are slanted or tipped over, this isconsidered to be a very good result. Referring back to the information recorded in
the logs helps us determine the most ideal conditions to consistently growproducts of similar (or better) quality.
Conclusion
So far, we have been able to define a rough “phase diagram” for thesynthesis of aligned ZnO nanorods at different levels of quality from numerous
experiments at various conditions. In the future, we hope to use this as afoundation for the large-scale production of perfectly aligned and uniform ZnOnanorods for application in nanodevices. Also in the future, our work may benefit

significantly from several important developments, including the development ofa more robust method of synthesis and a more precise furnace system. These
will enable us to achieve better consistency and have more control over thekinetics and conditions of the nanorod growth, as well as quality of the products.
Acknowledgements
I would like to take this chance to first thank Dr. Conrad for this precious
opportunity to participate in university research and to earn a little summer
money. I also would like to thank Jinhui for his care, guidance, knowledge, andpatience as my advisor. Dr. Wang is the man, and I admire his leadership of theresearch group. I want to give a high-five to Will for the opportunity to use theSEM, and a big-ten to Changshi for letting me take part in his research whichgave me the opportunity to further explore the topic of nanoscience and a rare
break for recognition. I pray for the best of everyone in Dr. Wang’s researchgroup, and I wish the graduate students good luck in efficiently obtaining theirdegrees. And last, but not least, I would like to thank MARTA and the Tech
Trolleys for being world-class transportation systems.
References(1.) Wang, Xudong. Patterned and Aligned ZnO 1D Nanostructures: Fabrication,Characterization, and Applications. School of Materials Science &Engineering, Georgia Institute of Technology.
(2.) Song, Jinhui; Xudong Wang; Elisa Riedo; Zhong L. Wang, The Journal of
Physical Chemistry B Letters, 2005, 109, 9869-9872.