本實驗利用脈衝雷射剝蝕凝聚之急熱急冷特性，於不同環境(大氣、真空和水中)，或是不同雷射功率條件下合成氧化鉻凝聚物，並利用X-光繞射、偏光顯微鏡、掃瞄式、穿透式電子顯微鏡、拉曼與霍氏紅外線光譜來研究其成分、顆粒大小、形狀、結構與內應力。
首先，利用在大氣環境下以雷射剝蝕鉻、矽順序疊置之靶材在充滿氧的氣氛下，合成含有4 at% Si的氧化鉻凝聚物。在穿透式電子顯微鏡觀察中，發現凝聚物主要為呈現六角板狀具剛玉結構之Si:α-Cr2O3，並且含有少量類尖晶石結構之Si: Cr3O4。其中六角板狀Si:α-Cr2O3奈米顆粒傾向以(0001)或是側邊(10-12)接合，並藉由布朗運動(Brownian rotation)，在界面處產生差排來調整界面，以達到平行磊晶的關係來降低界面能。
此外，在大氣中以相同實驗條件進行雷射剝蝕純鉻靶，利用穿透式電子顯微鏡探討此非等性晶體之形狀與內應力之間的關係。此實驗合成的單晶α-Cr2O3凝聚物主要為菱形且具有發達{01 2}面，並且有極少量呈六角板狀具有發達{11-20}面。利用拉曼光譜測得CrO6多面體之紅位移(red shift)估算其內應力可以高達5 GPa。仔細分析高解析影像，發現六角板狀之{11-20}具有局部壓應力，而在菱形{-1012}、{10-11}和(0001)則具有局部張應力。
接下來，改變雷射剝蝕環境，將純鉻靶在真空系統中，通以高純度氧氣，流量為20 sccm，合成較小的凝聚物。藉由穿透式電子顯微鏡觀察，發現這些急速冷卻的凝聚物為非晶質，且具有兩組面間距離0.259-0.266奈米和0.355-0.371奈米的波浪狀層狀結構，此兩組面間距離分別趨近剛玉結構α-Cr2O3之(10-14)和(11-20)，而此兩平面之CrO6八面體鍵結方式，分別為具有0和1個週期性強鍵(periodic bond chain)。此非晶質凝聚物經由電子束照射後，CrO6八面體會鍵結形成符合剛玉結構α-Cr2O3之(01-12)面的面間距離，並且進一步以(01-12)聚合(coalescence)以降低界面能。利用拉曼光譜測得CrO6多面體之紅位移估算其內應力可以高達4 GPa。
進一步在真空系統提高4個級數的雷射功率剝蝕純鉻靶，合成大小為100到200奈米，且形狀接近球形的氧化鉻顆粒(particulate)。 穿透式電子顯微鏡觀察指出，球形顆粒主要分為三種類型：(1) 具有(-1101)、(-1012)和(11-20)小面的α-Cr2O3單晶顆粒，(2) 具有球晶形狀之類尖晶石Cr3O4多晶，(3) 經由雷射輻射加熱造成再結晶的顆粒。 這些凝聚物之微結構與相的不同可能是因為Cr2+的溶質共伴效應(solute trapping)而造成相當程度的組成過冷(constitutional supercooling)所產生的。利用拉曼光譜測得CrO6多面體之紅位移估算其內應力大概3.4 GPa。
最後，以純鉻靶在水中進行雷射剝蝕凝聚，合成氧化鉻凝聚物。 穿透式電子顯微鏡指出，在水中合成的氧化鉻凝聚物主要為十二面體的類尖晶石Cr3O4，並且以[21-1]sp//[01-1]t；(011)sp//(100)t 類似Bain relationship，從尖晶石結構扭曲成為正方晶系的t-Cr3O4。t-Cr3O4的{101}雙晶關係可能是由正方晶的變形或是聚合現象造成。部份區域出現1.5×(200)和2×(211)序化超晶格(commensurate superstructure)，可能是由於Cr2+、Cr3+和H+在配位數四或六的週期性排列所造成。拉曼光譜指出此凝聚物具有高達5 GPa的內應力。 吸收光譜的紅位移可能是由內應力與Cr2+存在引起。

This thesis is about the synthesis and characterization (electron microscopy and spectroscopy) of chromium oxide condensates prepared by a dynamic process of pulsed laser ablation (PLA) or pulsed laser ablation in liquid (PLAL) regarding the composition, size, shape, structure and internal stress of the condensates under the influence of laser parameters and dopant.

Firstly, dense chromium oxide nanocondensates dissolved with ca. 4 atomic % Si, according to energy dispersive X-ray analysis, were fabricated by PLA on a clamped Cr/Si target in air purged with oxygen for a very rapid heating/cooling and hence pressure effect. Transmission electron microscopic observations indicated the predominant corundum-type Si4+:α-Cr2O3 nanocondensates are hexagonal in shape with significant internal compressive stress, and the minor spinel-like Si4+:Cr3O4 nanocondensates are octahedral in shape with considerable tetragonal distortion. The predominant Si4+:α-Cr2O3 condensates tended to coalesce over stepwise (0001) or lateral (1-102) surface to generate dislocations until parallel epitaxial relationship was exactly reached via a Brownian rotation process of the particles. X-ray diffraction indicated that the internal compressive stress was quite released for the coarsened/coalesced condensates. The laser ablation results in this part shed light on the condensation effect, as an alternative to a solidification process, on the formation of Cr-rich oxide particles on the surface of Cr4+ doped YAG fiber.

In addition, the α-Cr2O3 single crystal nanocondensates were fabricated by pulsed laser ablation in air purged with oxygen and characterized by analytical electron microscopy regarding shape dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed {01-12} surfaces and occasionally hexagonal plate with thin {11-20} edges and blunt corners. Such nanocondensates showed Raman shift for the CrO6 polyhedra, indicating a local compressive stress up to 5 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3-1.0 % strain) near the relaxed {-1012}, {10-11} and (0001) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating-cooling process.

Furthermore, amorphous chromium oxide nanocondensates were fabricated by energetic PLA on Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with a bimodal interspacing 0.259-0.266 and 0.355-0.371 nm which are close to that of specific lattice planes of the stable α-type structure, i.e. (-1104) and (01-12) having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains (PBC), respectively. Such amorphous nanocondensates were observed in-situ to became more polymerized by forming (01-12)-like layers and then fully crystallized as α-Cr2O3 for further (01-12)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient α-Cr2O3 yet at higher frequencies due to an internal compressive stress up to ca. 4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase.
Moreover, the chromium oxide condensates nearly spherical in shape ranging from 0.1 to 0.2 micron in diameter were fabricated by laser ablation on Cr target at a very high power density of 1.8×1012 W/cm2 for a very rapid heating and cooling effect. Analytical electron microscopic observations of such spherical particulates revealed three types: (1) α-Cr2O3 single crystal with (-1101), (-1012) and (1-210) facets, (2) spinel-like Cr3O4 polycrystals with spherulitic texture, i.e. a rather corrugated solidification front, (3) recrystallized Cr3O4 polycrystals derived from type 2 by radiant heating. The microstructure and phase difference among the particulates can be attributed to varied extent of supercooling under the influence of rather complicated Cr2+ solute trapping of the molten and solid phases in the Cr3O4-O pseudo-binary in vacuum. The chromium oxide condensates being spherical yet full of facets, with significant internal compressive stress up to ca. 3.4 GPa according to Raman shift, and with UV-absorbance close to violet light due to the presence of internal stress and/or Cr2+, may have potential optoelectronic and catalytic applications.

Finally, analytical electron microscopic observations indicated that the chromium oxide nanocondensates fabricated by PLAL (in water) are predominantly dodecahedral Cr3O4 with varied extent of tetragonal (t-) distortion from the spinel (sp)-type following the Bain relationship [21-1]sp//[01-1]t; (011)sp//(100)t. The t-Cr3O4 nanocondensates have {101}-twining due to tetragonal distortion and/or a coalescence event. The additional x(200) and 2x(211) commensurate superstructures can be attributed to periodic presence of Cr2+, Cr3+ and/or H+ in the 4- and/or 6-coordinated lattice sites with an internal compressive stress up to ~ 5 GPa according to X-ray photoelectron and vibrational spectroscopic evidences. The presence of internal stress and Cr2+ ion caused red shift of the UV absorbance, thus shedding light on potential optoelectronic applications of the Cr3O4 nanocondensates.

Preface…………………………………….............. I
論文提要(中)........................................................................................................ III
Abstract.............................................................................................................. ..VI
Content………………………………………………………............ X
List of figures……………………………………………………….................. XIV
List of table and appendixes……………………………………………..... XXIII

Chapter 1
Introduction of research background
1.1. Fabrication of chromium oxide nanoparticles and films............................. 1
1.2. Phase behavior of chromium oxide under high pressure............................ 2
1.3. X-ray photoelectron spectroscopy of chromium oxide…………………… 3
1.4. Shape and surface energetics of chromium oxide........................................ 3
1.5. Dense condensates via PLA and PLAL......................................................... 4

Chapter 2
Laser ablation condensation of defective Si4+- doped chromium oxide nanocrystals
2.1. Introduction……………………………………………………………….... 6
2.2. Experimental………………………………………………………………... 7
2.3. Results………………………………………………………………………. 8
2.3.1 XRD…………………………………………………………………….. 8
2.3.2 Optical microscopy……………………………………………………... 9
2.3.3 SEM……………………………………………………………….……. 9
2.3.4 TEM…………………………………………………………………….. 9
2.4. Discussion…………………………………………………………………... 11
2.4.1. Phase/shape selection and internal stress of the particulates and condensates……………………………………………….....................11
2.4.2. Defect generation of Si4+:α-Cr2O3 nanocondensates…………............13
2.4.3. Implications……………………………………………………….......14
2.5. Conclusions…………………………………………………………...........15
Figures………………………………………………………………………......16

Chapter 3
Shape dependent local internal stress of α-Cr2O3 nanocrystal fabricated by
pulsed laser ablation
3.1. Introduction………………………………………………………………... 24
3.2. Experimental……………………………………………………………..... 24
3.3. Results and discussion…………………………………………………...... 26
3.4. Conclusions……………………………………………………………….... 32
Figures…………………………………………………………………………... 36

Chapter 4
Condensation and crystallization of amorphous/lamellar chromium
sesquioxide
4.1. Introduction………………………………………………………………... 42
4.2. Experimental……………………………………………………………..... 43
4.3. Results…………………………………………………………………....... 45
4.3.1. TEM observations of individual condensates….…………………....... 45
4.3.2. Optical and structural observations of the deposit on silica glass…... 47
4.4. Discussion………………………………………………………………….. 48
4.4.1. Laser parameter dependence of metastable Cr2O3…………………... 49
4.4.2. Internal compressive stress of partially devitrified Cr2O3 condensate.. 50
4.4.3. Comparative relaxation/crystallization path of sesquioxide
condensates…………………………………………………………..... 52
4.5. Conclusions……………………………………………………………….... 54
Figures………………………………………………………………................... 56
Chapter 5
On the spherical chromium oxide particulates via pulsed laser ablation at a
very high power density in vacuum with a specified oxygen flow rate
5.1. Introduction……………………………………………………………....... 68
5.2. Experimental……………………………………………………………..... 69
5.3. Results…………………………………………………………………….... 71
5.3.1. TEM ...................................................................................................... 71
5.3.2. Spectroscopy…………………………………………………….……. 72
5.4. Discussion………………………………………………………………….. 74
5.4.1. Power density dependence of the PLA product………………………. 74
5.4.2. Constitutional supercooling of the particulates in the Cr3O4-O pseudobinary………………………………………………………………......
75
5.4.3. Cause of α-Cr2O3 single crystal with submicron size and spherical shape
………………………………………………………………………... 76
5.4.4. Cr2+ dependence of UV-absorbance………………………………….. 77
5.5. Conclusions……………………………………………………………….... 77
Figures…………………………………………………………………………... 80

Chapter 6
Microstructures, phase transformation and optical properties of spinel-like
Cr3O4 condensates by pulsed laser ablation in water
6.1. Introduction………………………………………………………………... 90
6.2. Experimental……………………………………………………………..... 92
6.3. Results…………………………………………………………………….... 93
6.3.1. TEM…………………………………………..….…………................ 93
6.3.2. Raman shift and FTIR……………………………………………….... 95
6.3.3. XPS and UV-visible absorption………………………………………. 96
6.4. Discussion………………………………………………………………….. 97
6.4.1. Effect of H+/OH- on the size, shape and phase identity of the condensates
………………………………………………………………………... 97
6.4.2. Defect chemistry and defect clusters of Cr3+ and H+ co-doped Cr3O4.. 98
6.4.3. Martensitic-like transformation of Cr3O4…………………………...... 99
6.4.4. Cr ion dependent optical absorbance in UV region………………… 100
6.5. Conclusions……………………………………………………………….. 100
Figures………………………………………………………………………..... 105
References…………………………………………………………………….. 116

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