Growth of Aligned SWNTs in Quartz Substrates Jie Liu Department of Chemistry Duke University April 10, 2011 Guadalupe Workshop
Carbon Nanotube Structure θ = tan 1 [ 3m /( m + 2n)] d = 2 ac c 3( n + nm + m 2 ) / π Min, OY.; Huang, J. L.; Lieber, C. M. Acc. Chem. Res. 2002, 35, 1018. Terrones, M. Annu. Rev. Mater.Res. 2003, 33, 419. 2
传统半导体器件性能受限于 p 型器件 CNT 400 160 >100,000 >100,000 0.4-2eV 半导体技术路线图委员会选择碳材料的主要原因是其超高的电子和空穴迁移率, 以及碳纳米管可能的环栅结构的应用 Slide Courtesy of Professor Lianmao Peng
Great Device Performance for both p and n type Devices Carbon Nanotube Field-Effect Transistors with Integrated Ohmic Contacts and High-κ Gate Dielectrics Honjie Dai Group Stanford University A Doping-Free Carbon Nanotube CMOS Inverter-Based Bipolar Diode and Ambipolar Transistor Lian-Mao Peng Group Peiking University
Remaining Challenges for Nanotube Devices
Control of Orientations --Electric Field Guided Growth Dai HJ et al. Appl. Phys. Lett. 2001, 79, 3155 Ernesto Joselevich* Charles M. Lieber, Nano Letters, 2002, 2 (10), pp 1137 1141
Control of Orientations --Gas Flow Controlled Growth S.M. Huang, X.Y. Cai and J. Liu, JACS, 125(19), 5636-5637 (2003). S.M. Huang, B. Maynor, X.Y. Cai and J. Liu, Advanced Materials, 15(19), 1651 (2003)
Early Works on Aligned Growth on Single Crystal Substrates Liu s Group, Duke, 2000 Joselevich s Group, Israel, 2004
Early Works on Aligned Growth on Single Crystal Substrates Growth of Surface aligned Carbon Nanotubes (a) Schematic diagram of a hexagonal cell of sapphire crystal. The planes of different cutting directions are labeled by various colors (b) Diagram of a nanotube sitting on the surface of sapphire along certain crystal direction (c) AFM image of aligned carbon nanotube array. J. Am. Chem. Soc. 127, 5294 (2005) Chongwu Zhou USC, 2005 Roger s Group, UIUC, 2005
Relation of Diameter of Nanotubes and Alignment 1:15 1:12 1:10 1:8 1:4 1:2 1:1 Catalysts: Getting bigger
Relation between Diameter of Nanotubes and Alignment
Mechanism
Cu catalytic growth of CNTs with EtOH as carbon source a b 1 mm 500 µm c d 5.0 nm 0.0 50 µm 200 nm
Semiconducting Aligned CNTs
Semiconducting Aligned CNTs
CNTs Grown on SiO2 Under Same Conditions do not Show Selectivity
Aligned CNTs on Quartz Under different Conditions also not Showing Selectivity
What is the Next Step? Increase the Current Density of Devices for High frequency devices; Density Increase; Diameter Control; Understanding.
SWNT Density Improvement and Current Enhancement Two-step CVD 0.05V bias Density and output current: 1.5~2 times increase John Rogers and et al. Adv. Mater. 2010, 22, 1826 1830
Low-pressure CVD and stacked multiple transfer Chongwu Zhou and et al. Nano Research ASAP 1V bias First growth: 15 tubes/μm; After four layer transfer: 55 tubes/μm; Highest on-sate current density of 92.4 μa/μm
How to further increase current density? Strategy I: Synthesis of SWNTs with large diameter Phaedon Avouris and et al. Nano Lett 2005, 5, 1497 Yu-Chih Tseng and et al. Nano Lett 2006, 6, 1364 Ideal contact can be formed between Pd and SWNTs with the diameter above 1.4 nm. Large diameter SWNT (>1.4 nm) can carry two orders of magnitude higher current than small diameter SWNTs (<1.0 nm).
Methane CVD 1.1± 0.4 nm Our ethanol CVD 1.2 ± 0.3 nm LP ethanol CVD 1.388 ± 0.457 nm Peter Burke and et al. Nano Res2008,1,158 Our methanol/ethanol CVD Jie Liu and et al. J. Am. Chem. Soc. 2008, 130, 5428. Chongwu Zhou and et al. Nano Res. ASAP Density is not very high. 1.4~1.8 nm with an average diameter of ~1.65 nm Jie Liu and et al. Nano Lett. 2009, 9,800-805
Strategy II: Increase the catalyzing efficiency of catalyst particles. First CVD Growth C 2 H 4, H 2 O, H 2 and Ar 800 o C ~ 850 o C Fe x O y nanoparticles C 2 H 5 OH, CH 3 OH, H 2 and Ar 900 o C Second CVD Growth Some catalyst particles still active after first CVD growth of CNT forests. Jie Liu and et al. J. Am. Chem. Soc. 2010, 132, 336-341.
Experimental Design: Multiple-Cycle Growth 900 o C Aim: activate more catalyst particles and synthesize largediameter SWNTs.
Experimental Results: Horizontally aligned array of SWNTs with highly dense packing Density is too high to count the number of SWNTs by SEM!
AFM Characterization The density of SWNT array is 20 ~ 40 SWNTs/μm, which is 3~4 times as high as the array density made by one-time growth. In some areas, SWNTs almost form a continual monolayer film. Even AFM is hard to distinguish the isolated SWNTs.
Raman Characterization RBM G band From quartz surface Raman spectra collected in the 16 spots. 633 nm laser.
HRTEM Characterization (transfer SWNT array to copper grid by using PMMA film) Total number : 120 Diameter distribution: 2.38 ± 0.48 nm Maximum: 3.73 nm; Minimum: 1.18 nm Only 3 SWNTs with the diameter below 1.4nm.
Mechanism? ne-time Growth Two-cycle Growth Three-cycle Growth Four-cycle Growth 4~ 10 SWNT/μm 10~20 SWNT/μm 20~ 40 SWNT/μm 2~ 5 SWNT/μm
Hypothesis: Increase the Probability of SWNTs Nucleation At the stage of carbon feeding rate increase, different size catalyst particles start to nucleate SWNTs due to different carbon feeding rate. However, not every particle can nucleate SWNT at this stage, even for same size nanoparticles. There is a nucleation probability problem. Multiple-cycle growth can greatly increase the chances of SWNT nucleation on catalyst particles.
Proof: new nanotubes grow out from catalyst area. First CVD Growth Second CVD Growth At the same time, short tubes were observed after second CVD growth, which indicates some old nanotubes were etched in the second growth.
Etching Effect Study: Keep Methanol and Ar as the Protection Gas in the Off-steps In order to further investigate the etching effect, we used methanol and Ar as the protecting gas instead of pure Ar in the off steps between two CVD growth steps in a threecycle growth. As shown in SEM images, very few nanotubes were grown on quartz substrate. It means most of nanotubes were etched by methanol.
Current Thinking Density increase of SWNT array is due to the higher efficiency of nucleation. This hypothesis is confirmed by comparison of one-time, two-cycle and three-cycle and four-cycle growth results and in-situ analyzing SEM images of the same catalyst area on a quartz wafer after the first growth and after the second growth. The existence of etching effect during the CVD growth process has been discovered. In the fourth growth cycle, the etching effect starts to dominate over the growth, then resulting in the decrease of SWNT density. We speculate that it is from OH group because oxygen atom in OH group has a weak oxidation function and can react with amorphous carbon. This etching effect is likely the reason that only large-diameter SWNTs were obtained in multiplecycle CVD growth.
Acknowledgement Funding ONR NRL(N00173-04-1-G902) Duke University People Dr. Lei Ding Dr. Weiwei Zhou Dr. Alex Tselev Dr. Dongning Yuan Professor Yan Li (Peiking University) and students