NANO LETTERS
Atomistic Stress Fluctuation at Surfaces and Edges of Epitaxially Grown Silver Nanorods
2004 Vol. 4, No. 7 1221-1224
Saw-Wai Hla,*,† Albert Prodan,‡ and Herman J. P. van Midden‡ Nanoscale & Quantum Phenomena Institute, Physics & Astronomy Department, Ohio UniVersity, Athens, Ohio 45701, and Jozˇef Stefan Institute, JamoVa 39, Ljubljana, SI-1000, SloVenia Received April 4, 2004; Revised Manuscript Received May 6, 2004
ABSTRACT Structural details of epitaxially grown silver nanorods on a β-MoTe2 surface have been investigated by scanning tunneling microscopy. Atomicscale surface bending and surface reconstruction are directly observed at nanorod surfaces and edges. In agreement with a theoretical prediction, a stress fluctuation is observed in the two topmost atomic layers of nanorods. Due to a tensile stress the surface layers are occasionally bent while the subsurface layer experiences a compressive stress resulting in a novel surface reconstruction.
Engineered nanoparticles represent a new class of materials whose structures and physical properties often differ from those in the bulk.1-17 An important property of anisotropic particles, such as nanowires/nanorods, is surface layer bending.7 A theoretical report18 suggested that a stress fluctuation should occur near the surface region of a small gold slab to accommodate the surface layer bending. To confirm this phenomenon and to check whether such behavior is valid in other cases as well, we have investigated structural peculiarities occurring at surfaces and edges of epitaxially grown silver nanorods on a (001) van der Waals surface of β-MoTe2. The experiments were performed under ultrahigh vacuum (UHV) conditions (pressure below 10-10 hPa) using a scanning tunneling microscope (STM) operated at room temperature (RT). Silver of 99.999% purity was deposited onto freshly cleaved β-MoTe2 samples held at RT by thermal evaporation. Then the samples were transferred to the STM chamber without exposing them to the atmosphere. We examined a series of samples with various silver coverages ranging between 0.2 and 20 monolayers (ML). The actual thicknesses of individual nanorods were determined from STM images, while the average layer thicknesses were estimated from the time periods needed to deposit thicker specimens under identical conditions. The β-MoTe2 surface layer consists of troughs formed by tellurium (Te) atoms with a 0.06 nm height corrugation * Corresponding author. E-mail:
[email protected], Web: www.phy.ohiou.edu/∼hla. † Ohio University. ‡ Joz ˇ ef Stefan Institute. 10.1021/nl049492u CCC: $27.50 Published on Web 05/25/2004
© 2004 American Chemical Society
(Figure 1a). It is known that silver grows on a (001) β-MoTe2 surface with its (211) surface plane parallel to the substrate.19 At the initial stages of the growth, silver forms small elongated chains of atoms along the β-MoTe2 [010] direction due to a preferential diffusion of silver. At a nominally 2 ML thick deposit, 20 to 50 nm wide, up to 300 nm long, and four to eight layers high, silver nanorods are formed (Figure 1b). The long axes of the nanorods are aligned parallel to the β-MoTe2 [010] surface direction. Figure 1c presents the length-to-width aspect ratio and volume of nanorods plotted as the functions of nanorod length. The topmost surface layers of most nanorods examined here are only partially filled, exposing the underlying layer structures. Thus, the structural changes at both the top and second topmost layers of the nanorods can be probed with the STM. Since most nanorods investigated in our experiment reveal similar structural properties, our report discusses various parts of a single nanorod shown in Figure 2. In Figure 2a, a three-dimensional (3D) STM image of the middle part of a silver nanorod on β-MoTe2 surface is shown. This nanorod is five atomic layers high and 35 nm wide. The fifth adlayer is only partly filled and composed of a narrow strip running along the middle part and a wider layer at the left side of the nanorod surface. Step-like edges are formed between the second and the fourth silver layers at the right edge and between the third, fourth, and fifth layers at the left edge of the nanorod. Details are revealed in Figure 2b, 2c, and 2d. The small area, marked as “1” in Figure 2a, is shown at a higher magnification in Figure 2b. Atomic resolution is
Figure 1. Substrate and the silver nanorods. STM images of a periodically corrugated tellurium (001) surface layer of β-MoTe2 (1.7 × 1.6 nm2, It ) 12 nA, Vg ) 10 mV) (a) and epitaxially grown silver nanorods aligned with their long axes parallel to the [010] direction of β-MoTe2 (120 × 180 nm2, It ) 1 nA, Vg ) 50 mV) (b). (c) The length-to-width aspect ratio (dark squares) and the volume (gray squares) plotted as functions of the nanorod lengths. The two lines indicate the linear fit of the data.
achieved at both the substrate tellurium layer (blue color) and the silver layers forming the nanorod edge (red and green colors). This image confirms that the nanorod is grown with its (211) surface plane parallel to the (001) β-MoTe2 surface19 (Figure 3). Here, we are interested in atomic structural details at the bent edge. Small colored arrows at the upper part of 1222
the Figure 2b indicate the positions of the topmost silver atom rows. According to the corresponding height scan, the rows indicated by the red and orange arrows are positioned well above their expected heights. In addition to their height variation, the distances between the atomic rows at this bentedge region are also larger than that of the adjacent reconstructed area, discussed further below. Figure 2c shows an enlargement of the area marked as “2” in Figure 2a. In addition to the substrate (red) and the fourth silver layer (at the right), the two incomplete adlayers are clearly resolved. The right one is formed of four ridges only. According to the line profile it is strongly bent. Thus it is positioned well above the left wider adlayer, which is flat, composed of thirteen ridges, and positioned at the proper height. The average spacing between its thirteen ridges is ∼0.64 nm and is slightly larger than the corresponding distance of the substrate tellurium troughs (0.63 nm). Further, the part of the height scan marked with the white box represents a reconstructed region with a two-up one-down periodicity. This region is shown again in Figure 2d, which is an enlargement of the region “3” in Figure 2a. The corrugation amounts to only about 0.01 nm and the atomic rows are displaced laterally as well. The average distance measured between the elevated pairs is 0.54 ( 0.01 nm, and the one between two adjacent rows at different heights amounts to 0.63 ( 0.01 nm. Thus, the average row spacing is only 0.60 ( 0.02 nm, which is even below the substrate periodicity at the substrate-to-deposit interface (0.63 nm). The epitaxial relationship between β-MoTe2 and the deposited silver was determined by transmission electron diffraction.19 Atomic ridges of the silver (211) contact plane are locked into the tellurium troughs of the substrate (001) plane, aligning the silver [01h1] direction with the [010] direction of β-MoTe2 (Figure 3). The relatively large lattice mismatch is accommodated by adjusting the interatomic distances in the deposit to those of the substrate, causing a tensile stress along the [01h1] direction and a compressive stress along the perpendicular [11h1h] direction. Thus, the growth of the nanorods described in this letter is definitely of the Volmer-Weber type. Bulk silver parameters, obtained from X-ray diffraction (XRD) experiments, are listed in Table 1 together with some experimentally determined values obtained from the atomically resolved STM images. Silver parameters were measured on the fourth and the incomplete fifth adlayers of different nanorods. Tellurium parameters, acquired far enough from the nanorods to be free of their influence, were measured for calibration purposes. The comparison between the measured data reveals that the average spacing between two silver atoms along the [01h1] direction in the fourth layer is expanded from its bulk value (0.29 nm) to 0.35 nm, while the one along the perpendicular [11h1h] direction is contracted from 0.71 nm in the bulk to about 0.64 and 0.60 nm in the unreconstructed and reconstructed regions, respectively. The values show that the substrate largely influences the interatomic distances a few layers into the deposit. In relation to the underlying fourth layer, the adlayer is certainly subjected to a tensile stress, which eventually results in bending of Nano Lett., Vol. 4, No. 7, 2004
Figure 2. (a) Three-dimensional STM image of a part of a silver nanorod at lower magnification (65 × 56 nm2, It ) 1 nA, Vg ) 50 mV). The green arrow indicates a well-defined step between the second and the fourth layer, and the white arrow shows a bent part of the fourth layer. (b) Enlarged image of the part, marked as “1” in (a); small colored arrows indicate the upper-most silver atom rows and the corresponding height scan reveals that the rows are positioned 0.08 and 0.11 nm above the last completed layer; the white and green arrows indicate the same places as those in (a). (c) An enlargement of part “2” in (a), showing atomic rows of two incomplete silver adlayers; the right one has a smaller width and is strongly bent. The bent right adlayer is positioned substantially above the wider left one. The part of the height scan marked with the white box highlights a reconstructed region with a two-up one-down periodicity. (d) An enlargement of the silver surface reconstruction, marked as “3” in (a). The arrows indicate the elevated rows. The corrugation height and the average distances are indicated. Table 1. Measured Unit Cell Parametersa a′ [Ag (211)] bulk parameters (XRD) fourth silver layer reconstructed surface fifth surface adlayer a
0.709 nm
b′ [Ag(211)]
a [Te (001)]
b [Te (001)]
0.289 nm 0.35 ( 0.01 nm
0.633 nm
0.347 nm
0.60 ( 0.02 nm 0.64 ( 0.01 nm
lattice mismatch
-15% -10%
Negative values mean compressed lattice distances.
the narrow strips. Thus, an obvious stress fluctuation is taking place in the topmost layers of the nanorods. The most suitable places for the surface restructuring to occur are regions close to the edges of the nanorods. Relaxation can be achieved in various ways. One possibility is shown in Figure 2b, where the fourth silver layer does not form a sharp cutoff edge, but rather a curved transitional region. The distances between the atomic rows are increased in such cases, and the entire region experiences a tensile stress. A second possibility is the one shown in Figure 2c and 2d. According to Tartaglino et al.,18 bending of a thin reconstructed (111) gold strip should result in a sharp oscillation in stress upon approaching the surface. A compressive stress is expected in the subsurface layer and a tensile stress should occur in the topmost layer. Unlike gold, silver surfaces are usually not reconstructed, but a Nano Lett., Vol. 4, No. 7, 2004
reconstruction might be induced by effects similar to the one caused by bending in the case of gold. In our investigation, the reconstructed areas were observed adjacent to the bent adlayers only indicating that the tensile stress in the adlayer induces a high compressive stress in the underlying layer and consequently reconstruction in the adjacent regions. In conclusion, an epitaxially grown nanorod represents a small silver strip with a gradual stress release toward its surface. However, this average behavior can obviously be altered in the last few surface layers. The present study reveals details about a novel stress phenomenon confined to specific surface regions of a silver nanorod. Studies of this kind are not only necessary for a better understanding of the surface processes but may also in the future also improve our ability to produce nanoparticles with engineered shapes 1223
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Figure 3. Nanorod-to-substrate epitaxial relationship. Both surface unit cells are indicated. Large black arrows indicate the compressive stress direction, and the wide striped arrows show the direction of the perpendicular tensile stress. The dotted arrows show the displacements in case of the two-up one-down surface reconstruction.
and orientations, which may eventually be usefully applied as elements in future nanotechnological processes. Acknowledgment. Financial support provided by the U.S. Department of Energy grant no. DE-FG02-02ER46012 (S.W.H.) and by a research grant of the Ministry of Education, Science and Sport of the Republic of Slovenia (A.P., H.J.P.vM.) is gratefully acknowledged.
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(12) (13) (14) (15) (16) (17) (18) (19)
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NL049492U
Nano Lett., Vol. 4, No. 7, 2004
