鎢(111)表面上的皺/平相變之研究

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姓名

陳龍輝(Lung-Hui Chen) 查詢紙本館藏

畢業系所

物理學系

論文名稱

鎢(111)表面上的皺/平相變之研究
(Faceting and defaceting phase transitions on W(111) surface)

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摘要(中)

在超高真空(約8 x 10-11 torr)的環境下，我們利用了可程式控溫低能量電子繞射(Temperature Programmed Low Energy Electron Diffraction, TPLEED)、可程式控溫歐傑能譜(Temperature Programmed Auger, TPA)和可程式控溫熱脫附(Temperature Programmed thermal Desorption, TPD)，來研究鈀(Pd)及氧(O)覆蓋層對於鎢(tungsten)基板的影響，亦即Pd/W(111)及O/W(111)表面系統中，溫度對表面結構變化的影響。第一部份的研究，是在Pd/W(111)的系統中，我們用TPLEED得到表面在高溫下的即時資訊，進而觀測到可逆的皺/平相變 : (111)/{112};另外由TPA中Pd的訊號，更進一步發現在產生皺/平相變的溫度下，當形成(111)平面時，沾附於表面的Pd，其厚度是會稍微減少至約1.2個物理單層(physical monolayer, PML);但在形成皺化{112}面時，Pd的厚度會稍微提升約0.1PML;更進一步我們發現在不同的溫度下退火(anneal)樣品，可以形成不同大小的皺化金字塔(faceting pyramid)，其中最大的金字塔所需退火的溫度恰在平(defaceting)相變發生之前;此外藉由分析在不同條件下製備的樣品，相變時由TPLEED觀察到的弛滯曲線(hysteresis loop)，我們發現在表面上的鈀三維島的密度(Pd 3-d island density)與相變發生的溫度有非常密切的關係。第二部份的研究，是在O/W(111)的系統中，我們觀察到了隨著溫度及氧(Oxygen)的覆蓋度(coverage)不同，在LEED上顯示出一系列不同的結構;同時我們也觀察到在這個系統下，存在著{110}及{112}方向的皺化(faceting)共存態;另外藉由TPLEED觀察高溫下的LEED影像，我們也對平衡晶體的形狀(equilibrium crystal shape)在定義上提出一些與最近報導的文章分岐的看法。

摘要(英)

We have used Temperature Programmed Low Energy Electron Diffraction (TPLEED) and Temperature Programmed Auger (TPA) and Temperature Programmed thermal Desorption (TPD) to study the effect of temperature on the surface structure of Pd on W(111) and oxygen on W(111) in an ultrahigh vacuum (UHV) chamber with a base pressure of 8 x 10-11 torr. In Pd/W(111) surfaces (as reported in Part I), we can get the real-time information of the surface at high temperature and then observe the reversible faceted/planar phase transition. Furthermore, TPA reveals that the apparent coverage of Pd changes across the transition. When the surface structure changes from faceted {112} to planar (111), the wetting layer of Pd slightly reduces to about 1.2PML (physical monolayer). Besides, the coverage of Pd slightly increases about 0.1PML when the surface structure changes from planar (111) to faceted {112}. Furthermore, we observe that the facet’s size depends on annealing temperature. The largest facet is obtained when annealing at the temperature right below the temperature of defaceting transition. By analyzing the hysteresis from TPLEED of the sample with different preparation conditions, we observe the temperature of the faceting/defaceting phase transition depends strongly on the density of Pd 3-d islands on the surface. In O/W(111) system (as reported in Part II), we observe a series of LEED patterns when we change the oxygen coverage and annealing temperature. In addition, we observe the coexistence of {110} and {112} facets from LEED images in this system. Besides, we observe some evidence about the equilibrium crystal shape (ECS) from TPLEED at high temperature, which is not consistent with the reported results in the recent literature.

關鍵字(中)

★ 相變
★ 皺化
★ 鎢

關鍵字(英)

★ phase transition
★ faceting
★ tungsten

論文目次

Part I
Faceting / defaceting phase transition
in the Pd/W(111) system
1.Introduction…………………………………………2
2.Experimental…………………………………………4
2.1 Chamber and tools…………………………….4
2.2 Sample preparation and temperature
measurement…………………………………….4
2.3 Characteristics in our UHV system……….4
2.4 Deposition of Pd and coverage
calibration…………………………………….6
3.Results……………………………………………….9
3.1 Phenomenon of reversible
faceting/defaceting phase transitions….9
3.1.1 Phase transition as seen by
TPLEED………………………………………9
3.1.2 Phase transition as seen by
TPAuger……………………………………12
3.1.3 Correspondence between TPLEED and
TPAuger……………………………………13
3.2 Apparent thickness of Pd on W(111)…….15
3.2.1 Wetting layer of Pd on W(111)………15
3.2.2 Three-dimensional Pd islands……….17
3.3 Dependence of the dip’s depth on sample
preparation……………………………………18
3.4 Dependence of the facet’s size on
annealing temperature………………………19
3.5 Dependence of the transition path on the
number of heating cycles:the first cycle
vs. later cycles…………………………….21
3.6 Weak dependence of the transition path on
the addition of Pd………………………….22
3.7 Dependence of hysteresis shape on the
sample temperature during deposition….23
3.8 Weak dependence of the defaceting
transition path on the heating
rate for later cycles………………………25
3.9 The change of hysteresis loops after
multiple heating cycles……………………27
3.10 The quenched surface after a prolonged
heating treatment………………………….28
4.Discussion………………………………………….30
4.1 Controlling of the facets
size…………………………………………….30
4.2 The intrinsic path of hysteresis……….31
4.3 The significance of 3-D islands…………32
5.Conclusion………………………………………….38
6.Reference……………………………………………39
Part II
Coexistence of {110} and {112} facets
in the O/W(111) system
1.Introduction……………………………………….41
2.Experimental……………………………………….42
3.Results………………………………………………43
3.1 Various phases of the O/W(111)surface…43
3.2 Comparison of the LEED images with the
results of simulation LEED program…….45
3.3 Some features of the coexistent
system………………………………………….46
3.4 Studying this system at high temperature
by TPA and TPD……………………………….47
3.5 TPLEED result of a faceted surface at high
temperature……………………………………48
4.Discussion………………………………………….49
4.1 Coexistent states of {112} and {110}
faceting……………………………………….49
4.2 Desorption of oxygen from the W(111)
surface…………………………………………50
4.3 Comparison of the TPLEED image with the
ECS results reported in the literature.51
5.Conclusion………………………………………….52
6.Reference……………………………………………53

參考文獻

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指導教授

宋克嘉、粘正勳
(Ker-Jar Song、Cheng-Hsun Nien)

審核日期

2005-7-21

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