Reciprocal-lattice Space Imaging of X-ray Intensities Diffracted from Nanowires Yoshimoto Res. Group, Tokyo Institute of Technology [3]. We used two samples that were similar except for the nanowires’ orientations; in addition, both substrates had a miscut of approximately 0.1 . Sample A had nanowires almost perpendicular to the sapphire [1 0 1 0] direction; and sample B had nanowires nearly parallel to the [1 0 1 0] direction. The coordination is expressed using a hexagonal symmetry. The samples were grown using a pulsed laser ablation method. The surface morphology of sample A was observed by atomic force microscopy (AFM) in air (Fig. 1). The nanowires were found to be of the order of mm in length, which was surprisingly long, by scrutinizing hundreds of successive AFM images (scan size 10 μ m × 10 μ m, not shown here). The images indicate that the nanowires were grown along step edges and were approximately 20 nm in width and 0.5 nm (two atomic planes) in height. The density was a nanowire per 50 nm. H ere, the nanowires that we are interested in are very thin, narrow, and long. If the wires are 1D crystalline, the scattering pattern or diffraction domain (the Fourier transform) in the reciprocal-lattice space shows sheets (which are perpendicular to the wires and whose diameters are inversely proportional to the width s of the wires). Let us assume that the sheets elongate along the sample surface normal from bulk- crystal Bragg points due to a surface truncation effect. The Ewald sphere can simultaneously intersect some of the sheets accordingly (Fig. 2). We used possibly high-energy X-rays to excite many diffracted X-rays. Measurements were performed at the undulator Fig. 1. AFM image (0.4 μ m × 0.4 μ m) of the nanowires grown on an ultrasmooth sapphire surface of sample A. nm nm 0.50 0.00 100 200 300 400 400 300 200 100 0 nm Structural investigation is essential for development of nanotechnological materials such as nanoelectronics and nanophotonics. X-ray diffraction/scattering is one of the most promising non-destructive methods for observing atomic-scale structures and morphology of the nanomaterials. A nanowire structure on a device surface is particularly interesting since this wire could be one of the minimum parts of electrical devices. The conventional approach for determining such a structure map s X-ray diffraction/scattering intensities under Bragg conditions in a reciprocal-lattice space. This provides quantitative information on the strain in the nanowires [1]. One of its disadvantages is its being time-consuming. In order to establish a rapid characterization method, we have developed an “obvious-at-a-glance analysis” technique for nanocrystalline structures [2]. By taking advantage of high-energy and brilliant X-rays of SPring-8, rapid measurements are achieved by mapping many diffracted intensities under Bragg conditions in the reciprocal-lattice space simultaneously. The present method uses many sets of limited diffracted intensities around a Bragg point and coaxes forth specific structural information such as crystallographic domain sizes. In this work, the method was successfully applied to observe sheet-shape diffraction emanating from ultrathin NiO wires and scattering arising from the 20 nm periodicity of stripes of nanowires. Specimens had the stripes of ultrathin NiO nanowires deposited on an ultrasmooth sapphire (0001) surface with two-atomic-plane step edges at Fig. 2. Geometry of grazing-angle high-energy X-ray scattering. Reciprocal-lattice space Diffracted X-rays Ewald sphere Crystal truncation sheet Bulk Bragg point Nanowire // Sample surface Incident X-rays Real space 39 Fig. 3. Pattern diffracted from nanowires nearly perpendicular to the incident X-ray beam. The distance from sample A to the imaging plate (camera length) was 190 mm. A pair of of numbers h, k stands for an index of a crystal truncation sheet. The lengths used for unit vectors a (// [ 1 0 1 0]), b (// [0 1 1 0]), and c (// [0 0 0 1] were 0.476, 0.476, and 0.421 nm, respectively. References [1] I.K. Robinson et al. : Phy s. R ev . Lett . 88 (2002) 096 1 04- 1. [ 2 ] O . Sakata, M . Takata, H . Suemat s u, A . Mat s uda, S . Ak ib a, A . Sa s ak i a n d M . Y os h i m o t o : Appl . Phy s. Lett . 84 (2004) 4239 . [ 3 ] M . Y os h i m o t o et al. : Appl . Phy s. Lett . 67 ( 1 995) 26 1 5 . [ 4 ] O . Sakata et al. : Surface R ev . Lett . 10 (2003) 543 . [ 5 ] H . H on g et at. : R ev . Sc i. Ins trum . 71 (2000) 3 1 32 . O s am i Sakata SPr in g-8 / JAS RI E-ma i l: o - s akata@ s pr in g8 .o r . jp b eaml in e BL13XU f o r s urface a n d in terface s tructure determ in at ion [ 4 ]. The exper i me n tal s etup in cluded a pa i r o f s l i t s f o r l i m i t in g the b eam si ze t o 0 .1 mm × 0 .1 mm a n d a n X-ray i mag in g plate f o r rec o rd in g d i ffract ion. A n X-ray b eam w i th a ph o t on e n ergy o f 25 keV wa s in c i de n t on the s ample at a n a n gle o f ar o u n d 0 .1 . The typ i cal exp os ure t i me wa s 1 0 m in. A si m i lar exper i me n tal arra n geme n t wa s u s ed f o r s urface s tructural obs ervat ion b y H on g et al. [ 5 ]. They ut i l i zed 20 keV X ray s a n d a CCD camera t o detect d i ffracted X-ray s ; the i r s ample S i wa s l o cated in ultrah i gh vacuum . F i gure 3 s h o w s the d i ffract ion patter n rec o rded whe n X ray s were in c i de n t alm os t perpe n d i cular t o the N i O n a no w i re s. D i ffract ion i mage s ( in recta n gle s ) p oin ted d o w n at a d i rect b eam p osi t ion. Whe n X-ray s were in c i de n t n early parallel t o the n a no w i re s , d i ffract ion i mage s l oo ked l i ke c i rcular arc s r o u n d the d i rect b eam p osi t ion ( no t s h o w n here) . Blu n t o r d i m i mage s were pr o duced b y X-ray thermal d i ffu s e s catter in g o f the s u bs trate cry s tal . We in dexed cry s tal tru n cat ion s heet s u sin g a c on ve n t ion al no tat ion expre ss ed f o r cry s tal tru n cat ion r o d s. Here, the in d i ce s h a n d k are def in ed b y h d i rect ion // [1 0 1 0 ] a n d k d i rect ion // [ 0 1 1 0 ] , re s pect i vely . Actually, the i mage s exte n d al on g the h a n d l d i rect ions ; here, the cl os e s t in teger h value s are s h o w n. The d i ffract ion i mage s have in te nsi ty pr o f i le s (f o r example, the ins et in F i g . 3) . Fr o m the ge o metr i cal c onsi derat ion in the rec i pr o cal-latt i ce s pace, the full-w i dth-at-half-max i mum (FWHM) cry s tall o graph i c d o ma in si ze s acr oss the n a no w i re s are 7 . 5 a n d 7 . 2 n m f o r s ample s A a n d B, re s pect i vely, ob ta in ed u sin g the in -pla n e FWHM o f the in te nsi ty pr o f i le . The average n a no w i re- n a no w i re s pac in g o f 46 n m f o r s ample A wa s e s t i mated fr o m the peak-t o -peak d is ta n ce . The value is n early equal t o that (~ 50 n m) ob ta in ed u sin g the AFM i mage . By further a n aly sis o f the d i ffract ion i mage s , the f o ll o w in g s tructural a n d m o rph o l o g i cal in f o rmat ion c o uld b e ob ta in ed: the s tra i ght n e ss o f the n a no w i re s ( in o ther w o rd s , the d is tr ib ut ion o f the n a no w i re w i dth), s pat i al d is tr ib ut ion o f the n a no w i re s , a n d at o m i c- s cale r o ugh n e ss o f the si de s urface o f the w i re s. The meth o d de s cr ib ed here w i ll all o w u s t o determ in e the s tructure s o f the n a no w i re s b ur i ed in the in terface s b etwee n a th in f i lm a n d a cry s tall in e s u bs trate . Furtherm o re, i t c o uld b e appl i ed t o s tructure a n aly sis dur in g gr o wth o f n a no w i re s a n d t o rap i d obs ervat ion o f a s urface pha s e tra nsi t ion in a n y atm os phere . In s ummary, we dem ons trated the fea sibi l i ty o f a n ew X-ray s catter in g meth o d f o r s tructure a n aly sis o f n a no w i re s in a i r u sin g a c o m bin at ion o f graz in g- in c i de n ce s urface s catter in g a n d h i gh-e n ergy s y n chr o tr on The si ze o f X-ray X-ray s. cry s tall in e d o ma ins in the n a no w i re s wa s evaluated . We ea si ly determ in ed whether the n a no w i re s are cry s tall in e o r am o rph o u s b y graz in g-a n gle h i gh e n ergy X-ray s catter in g . The meth o d ha s al so b ee n pr o ve n fea sib le f o r rap i d c on f i rmat ion o f the cry s tall o graph i c o r i e n tat ion a n d ep i taxy o f the n a no w i re s. 40
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