Creation of nanocrystals through a solid-solid phase transition induced by an STM tip

Science

Volumn 274, Issue 5288, 1996, Pages 757-761

  Zhang, Jian ^a   Liu, Jie ^a   Huang, Jin Lin ^a   Kim, Philip ^a   Lieber, Charles M ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

HELIUM; TANTALUM;

ARTICLE; CRYSTAL; CRYSTAL STRUCTURE; CRYSTALLIZATION; ELECTRIC FIELD; MAGNETISM; MOLECULAR PHYSICS; PRIORITY JOURNAL; SCANNING TUNNELING MICROSCOPY; X RAY DIFFRACTION;

EID: 0029858196     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.274.5288.757     Document Type: Article

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16. In the modification procedure, we used several methods. In one, the feedback loop was opened, the tip was advanced a preset amount toward the surface, and then the bias voltage was pulsed or ramped to the desired value. Alternatively, we allowed the z-loop to oscillate (by increasing the gain) for a short time (≤s 100 ms) at the desired voltage value. We were able to create controlled T-phase nanocrystals using both of these methods.
17. The success probability depends on the magnitude of the applied bias voltage and the tip and tunneling junction conditions. At bias voltages exceeding ∼1.8 V, the success probability for nanocrystal formation is on the order of 90%; this probability drops to zero at the modification threshold voltage, ∼1.2 V. These values exhibit uncertainty that reflects variations in tip structure and junction conditions.
18. The model in Fig. 3A illustrates the motion of Se atoms in response to a repulsive force that is expected when a negative bias voltage is applied to the STM tip. The Se atom vacancies predicted by our model (with a negative bias) have not yet been observed, although they are expected to be difficult to detect for several reasons. First, the percentage of vacancies is small; we estimated <1% vacancies in a single 40-nm T-phase crystal. Second, it is intrinsically difficult to observe all of the atomic sites in T-phase single crystals or nanocrystals because of the strong CDW modulation. In addition, this same model predicts that Se adatoms would be produced when a positive bias voltage was used to drive the transformation (because the Se atom motion would be toward the tip). The small number of adatoms generated in this reverse direction will not be readily observed because they would be expected to exhibit significant mobility on the surface.
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30. The reported CDW wavelength dispersions correspond to the standard deviation calculated from experimental images. We digitized the images to locate the CDW maxima, and then we used the distances between adjacent maxima to calculate the average and uncertainty in wavelength. The observed experimental uncertainty in roughly infinite crystals was 0.6 Å.
31. We thank E. Kaxiras and F. Spaepen for helpful discussions. The work of C.M.L was supported under NSF award DMR-9306684.