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Self-Organized Nanocavity Arrays on Pt/Ge(001) Avijit Kumar, Bene Poelsema, and Harold J. W. Zandvliet* Physics of Interfaces and Nanomaterials, Mesaþ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ABSTRACT: We report on the formation of well-ordered arrays of nanometer-sized cavities on a Pt-modified Ge(001) surface. The nanocavities are thermodynamically stable and form ordered domains separated by antiphase boundaries. When imaged at negative sample bias, the nanocavities exhibit an elliptical shape, whereas they appear as rectangular cavities at positive sample bias. A well-defined electronic state at 0.7 eV above the Fermi level is found. Spatial maps of the differential conductivity reveal that this electronic state is located at the regions between neighboring nanocavities in the well-ordered arrays. These arrays of nanocavities can serve as a template to study single molecules or molecular assemblies for future molecular electronics applications.
’ INTRODUCTION As down-scaling of electronic devices is reaching the atomic limit, a thorough and detailed study of the properties of single molecules and nanostructures is an absolute necessity. A proper understanding will pave the way to novel applications in the field of molecular electronics, nanoelectronics, and quantum computing. At the nanometer length scale, conventional lithographic processes are not suited to produce nanotemplates or arrays for the immobilization and study of individual molecules.1 Therefore, it becomes important to explore material systems or processes for fabricating functional nanotemplates where single molecular entities can be trapped. Such material systems are also relevant for providing nucleation centers, which can be beneficial for highly selective catalysis applications.2 This is one of the key reasons why we have been witnessing a large number of studies on self-assembly and self-organized growth of nanostructures during the last two decades.1,38 The realization of self-assembly of nanostructures using chemical synthesis using elementary building blocks (tectons) has revealed a cornucopia of new and exciting science.4,9,10 The weak noncovalent interactions among the tectons can be tuned to yield tailored structures and patterns with desired properties. However, in the case of self-organized growth of nanostructures using inorganic materials, such as metals and semiconductors, the scope and control for tailoring these structures become rather limited because these materials exhibit very strong directional (semiconductors) or nondirectional bonds (metals).3 This poses a difficulty in guiding the interatomic interactions leading to organized structures. Some examples of semiconductor or/and r 2011 American Chemical Society
metal self-organized structures are nanoripples on metals,11,12 nanogrooves on Cu(001),13 quantum dots of Ge on Si(001),3 and arrays of atomic wires on Ge(001).6,14,15 Here, we report the self-organized growth of ordered arrays of nanometer-sized cavities on a Pt-modified Ge(001) (Pt/Ge(001)) surface. The nanocavities are thermodynamically stable and exhibit structural and electronic properties that deviate substantially form the bare Ge(001) substrate. The Pt/Ge(001) surface has been studied for quite sometime because of its interesting electronic and physical properties. The surface is prepared by depositing a fraction of a monolayer of Pt on a clean Ge(001) surface. The sample is subsequently annealed, and the surface exhibits three phases: R terraces, β terraces, and nanowires on β terraces. More detailed information regarding these three phases can be found in refs 6, 16, and 17. Because of its appealing structural and electronic properties, this system has also been used as a template to study single molecules for molecular electronics applications.1820 Recently, we found a new phase consisting of well-ordered and densely packed arrays of nanocavities. The nanocavities are similar to the cavities observed by Fischer et al.6 However, in our case, the density of nanocavities is much higher and they are closely packed into well-ordered arrangements, which give rise to new electronic effects. Given the multiphase character and the rich properties of the Pt/Ge(001) surface, it can provide a unique template for the study and characterization of single molecules or Received: December 31, 2010 Revised: March 2, 2011 Published: March 18, 2011 6726
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nanoclusters by controllably tailoring the phase and thereby the electronic and structural properties of the surface. It is the aim of the present paper to report on the formation of densely packed and well-ordered arrays of nanocavities on the Pt/Ge(001) surface. We also have studied the physical, structural, and electronic properties of this new phase.
’ EXPERIMENTAL SECTION The experiments were performed in ultrahigh vacuum (UHV) at room temperature and at 77 K in an Omicron low temperature scanning tunneling microscope (LT-STM). The Ge(001) samples were cut from a nominal 76.2 mm (3 in.) by 0.5 mm, singleside-polished, n-type, Sb-doped wafer with a resistivity of less than 0.01 Ω 3 cm. The samples were mounted on a Mo holder, and any contact between the samples and other metals was carefully avoided. The preparation of the samples was carried out in a preparation chamber at UHV conditions. The samples were cleaned by sputtering with Arþ ions with an energy of 800 eV and subsequently annealed at a temperature of 1100 ((50) K. We repeated this cycle several times until we observed an ordered pattern of coexisting (2 1) and c(4 2) domains.21 Subsequently, we deposited 0.20.5 ML (monolayers) of Pt on this substrate, followed by prolonged annealing at 1050 ((25) K. After cooling to room temperature, the samples were transferred to the STM for imaging. The measurements were carried out at room temperature and at 77 K using electrochemically etched W-tips. Spatial maps of the differential conductivity were obtained using a lock-in amplifier. The modulation voltage was set to an amplitude of 1020 mV, and the frequency was 8.4 kHz. ’ RESULTS AND DISCUSSION Scanning tunneling microscopy (STM) measurements were performed on the Pt/Ge(001) samples that were prepared following the procedure described in the Experimental Section. Filled-state images recorded at room temperature revealed that the surface contained ordered arrays of nanocavities. Subsequently, the sample was cooled down to 77 K and imaged again. Figure 1 shows an STM image of such an area with a size of 80 80 nm2 recorded at a sample bias of þ1.5 V and a tunneling current of 0.3 nA. The surface exhibits several domains of wellordered and densely packed nanocavities, as shown in the outlined regions. The antiphase boundaries mainly run along the [310] and [310] directions, as indicated by the arrows in Figure 1. The inset of Figure 1 shows an STM image of nanocavities existing along with several Pt nanowires on the same terrace. However, the amount of nanowires found on the surface was pretty low. A nanowire that terminates near a nanocavity is indicated by a small circle. Apart from the nanowires and the nanocavities, the Pt/Ge(001) surface also consists of R terraces and β terraces.6,17 The formation of R and β terraces and nanowires is attributed to the amount of Pt that is locally available on the surface.17 For Pt coverages less than 1/4 of a monolayer, the surface is covered with R and β terraces, and only a small fraction of the β terraces is decorated with Pt nanowires. However, with increasing Pt coverage, the relative amount of β terraces grows at the expense of R terraces and a larger fraction of the β terraces is decorated with Pt nanowires.17,22 The difference between filled- and empty-state STM is substantial, as shown in Figure 2. For negative sample biases (0.8 V), the nanocavities exhibit an elliptical shape, as shown in Figure 2A, whereas at positive sample biases (þ1.0 V), the nanocavities have a
Figure 1. STM image of an ordered pattern of nanocavities on Ge(001) recorded at 77 K with a sample bias of þ1.5 V and tunneling current of 0.3 nA. The area of the image is 80 80 nm2. Several ordered domains are outlined. The arrows indicate [310] and [310] directions. The inset shows a region that contains nanocavities as well as a few nanowires (set points: 1.5 V, 0.3 nA; area: 15.8 15.8 nm2). A nanowire that terminates near a nanocavity is indicated by a small circle.
Figure 2. (A) Filled-state (set points: 0.8 V, 0.3 nA) and (B) emptystate (set points: þ1.0 V, 0.3 nA) images of the nanocavities recorded at 77 K. The image size is 13.5 13.5 nm2. The regions between the lateral edges of adjacent nanocavities, the antiphase boundaries, and the small protrusions at the edges of the nanocavities are outlined by small circles, large rectangles, and arrows, respectively.
rectangular shape, as depicted in Figure 2B. As is clear from Figure 2, the size distribution of the nanocavities is very narrow. The longitudinal and lateral axes dimensions of the ellipses are approximately 1.60 ( 0.05 and 0.54 ( 0.05 nm, respectively. However, at positive bias, the length and the width of the rectangular units are 1.60 ( 0.05 and 0.60 ( 0.05 nm, respectively. We found that, at low positive bias (0.2 V), the electronic shape transition as a function of sample bias of the nanocavities sets in. Line scans taken across the nanocavities reveal that they are only one atomic layer deep. A careful observation reveals two small protrusions, appearing faintly at the opposite longitudinal edges of the rectangular nanocavity. A few of these protrusions are indicated by arrows in Figure 2B. These protrusions are absent in filled-state images. It is frequently observed that, when two or more nanocavities are sufficiently close, they merge together to form a longer cavity. However, it is very rare that nanocavities merge together along their lateral axis direction, that is, in a direction perpendicular to substrate dimer rows. 6727
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Figure 3. Differential conductivities (dI/dV) recorded at 77 K at the regions located between neighboring nanocavities in the well-ordered and densely packed arrays (solid line) and at the antiphase boundaries (dashed line) of the ordered domain pattern (set points: 0.8 V, 0.3 nA). The peak located at 0.7 V corresponds to an electronic state located at the regions between neighboring nanocavities in the well-ordered and densely packed arrays.
Figure 4. (A) Topography and (B) derivative (dI/dV) map recorded at 77 K (set points: 0.8 V, 0.3 nA). Panel (C) shows a three-dimensional view of the superposition of the derivative map (B) onto the topography image (A). The size of each image is 16.2 11.8 nm2.
To investigate the electronic density of states of the nanocavity arrays in more detail, we performed differential conductivity measurements. Figure 3 shows the differential conductivity (dI/ dV) curves recorded at regions between the neighboring nanocavities in the densely packed areas and at antiphase boundaries (circles and rectangles in Figure 2B, respectively). The solid line corresponds to the differential conductivity, dI/dV, at regions between neighboring nanocavities, whereas the dashed line corresponds to antiphase boundaries. The curves are quite similar except for the fact that a strong peak at 0.7 V is present in the regions between neighboring nanocavities. We also recorded derivative (dI/dV) maps to investigate the spatial dependence of this electronic state at the surface. Figure 4 shows
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Figure 5. The image on the left is recorded at 77 K (set points: þ1.5 V, 0.3 nA; area: 14.7 13.2 nm2) and shows ordered clusters appearing regularly at the longitudinal boundaries of the nanocavities. The PtGe and GeGe dimers are found at the antiphase boundaries, which are indicated by smaller ellipses. The right image is a detailed view of the rectangular box in the left image. The two different types of atomic clusters are outlined by small circles.
the (A) topography and (B) derivative map and (C) a superimposition of the derivative and topography maps (set points: þ0.8 V, 0.3 nA). The topography and derivative maps were recorded simultaneously so that any drift between the two measurements is avoided. Figure 4C shows an ordered array of bright features (high electronic density) located at the regions between the lateral edges of adjacent nanocavities. Other systems exhibiting spatially confined electronic states on comparable length scales are, for instance, graphene on ruthenium23 and dicarbonitriles-sexiphenyl molecules on Ag (111).24 As has been pointed out in previous studies,6,24 the Pt nanowires form in the troughs of the substrate dimer rows of Ge(001). This is also evident from the inset of Figure 1, where a nanowire terminates in the middle of a trough. These troughs are formed when two adjacent nanocavities merge together in the direction of their longitudinal axis. The antiphase boundaries of the ordered domains of nanocavities consist of dimer rows similar to the β terrace, as shown by the small ellipses in Figure 5. The dimer rows are composed of GeGe and GePt dimers in a 1:1 ratio.25 At first glance, the regions between and around the densely packed nanocavities seem to consist of dimer rows as well. However, Figure 5, which is recorded at a sample bias of þ1.5 V, clearly reveals that these regions consist of ordered clusters of atoms, that is, tetramers or even larger units, rather than normal dimers. The ordered atomic clusters appear regularly at the longitudinal boundaries of the nanocavities, as shown by the large ellipses. The right-hand image is an enlarged zoom of the region outlined in Figure 5. The ordered asymmetric clusters are indicated by small bright circles. They appear to be rotated with respect to each other, similar to the tetramers of the Ge/Ag(001) system, as reported by Oughaddou et al.26 At the regions between neighboring nanocavities, there are also four geometrically symmetric bright spots that constitute another type of cluster, as outlined in the righthand image of Figure 5 by small dim circles. These clusters appear to be different from the asymmetric ones in the sense that they exhibit a pronounced electronic state at 0.7 V. Despite the high-resolution STM images, we are unable to put forward a structural model for the ordered nanocavity domains.
’ CONCLUSION We report on the formation of well-ordered and densely packed arrays of nanocavities on a Pt/Ge(001) surface. The 6728
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The Journal of Physical Chemistry C nanocavities appear as ellipses for negative sample biases and as rectangles for positive sample biases. The region between neighboring nanocavities in the densely packed arrays exhibits a well-pronounced electronic state at 0.7 V. The well-ordered structure between the nanocavities cannot be explained by dimers alone and requires the incorporation of more complex atomic arrangements as well.
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(25) Oncel, N.; van Houselt, A.; Huijben, J.; et al. Phys. Rev. Lett. 2005, 95, 116801. (26) Oughaddou, H.; Gay, J. M.; Aufray, B.; et al. Phys. Rev. B 2000, 61, 5692.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We would like to thank The Netherlands Organization for Scientific Research (NWO/CW ECHO.08.F2.008) for financial support. ’ REFERENCES (1) Berner, S.; Corso, M.; Widmer, R.; et al. Angew. Chem. 2007, 46, 5115. (2) Shiju, N. R.; Guliants, V. V. Appl. Catal., A 2009, 356, 117. (3) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (4) Otsuki, J. Coord. Chem. Rev. 2010, 254, 2311. (5) Spillmann, H.; Kiebele, A.; Stohr, M.; et al. Adv. Mater. 2006, 18, 275. (6) Fischer, M.; van Houselt, A.; Kockmann, D.; et al. Phys. Rev. B 2007, 76, 245429. (7) Dimitriev, A.; Lin, N.; Weckesser, J.; et al. J. Phys. Chem. B 2002, 106, 6907. (8) Lin, N.; Payer, D.; Dimitriev, A.; et al. Angew. Chem., Int. Ed. 2005, 44, 1488. (9) Jurow, M.; Schuckman, A. E.; Batteas, J. D.; et al. Coord. Chem. Rev. 2010, 254, 2297. (10) Liang, H.; He, Y.; Ye, Y. C.; et al. Coord. Chem. Rev. 2009, 253, 2959. (11) van Dijken, S.; Jorritsma, L. C.; Poelsema, B. Phys. Rev. B 2000, 61, 14047. (12) Dos Santos Claro, P. C.; Castez, M. F.; Schilardi, P. L.; et al. ACS Nano 2008, 2, 2531. (13) van Dijken, S.; de Bruin, D.; Poelsema, B. Phys. Rev. Lett. 2001, 86, 4608. (14) Wang, J.; Li, M.; Altman, E. I. Phys. Rev. B 2004, 70, 233312. (15) van Houselt, A.; Fischer, M.; Poelsema, B.; et al. Phys. Rev. B 2008, 78, 233410. (16) Gurlu, O.; Zandvliet, H. J. W.; Poelsema, B.; et al. Phys. Rev. B 2004, 70, 085312. (17) Gurlu, O.; Adam, O. A. O.; Zandvliet, H. J. W.; et al. Appl. Phys. Lett. 2003, 83, 4610. (18) Kockmann, D.; Poelsema, B.; Zandvliet, H. J. W. Nano Lett. 2009, 9, 1147. (19) Saedi, A.; Berkelaar, R. P.; Kumar, A.; et al. Phys. Rev. B 2010, 82, 165306. (20) Berkelaar, R. P.; S€ode, H.; Kumar, A.; et al. J. Phys. Chem. C 2011, 115, 2268. (21) Zandvliet, H. J. W.; Swartzentruber, B. S.; Wulfhekel, W.; et al. Phys. Rev. B 1998, 57, 6803. See also: Zandvliet, H. J. W. Phys. Rep. 2003, 388, 1. (22) Zandvliet, H. J. W.; van Houselt, A.; Poelsema, B. J. Phys.: Condens. Matter 2009, 21, 474207. (23) Zhang, H. G.; Hu, H.; Pan, Y.; et al. J. Phys.: Condens. Matter 2010, 22, 302001. (24) Klappenberger, F.; Kuhne, D.; Krenner, W.; et al. Nano Lett. 2009, 9, 3509. 6729
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