Growth of silicon nanostructures on graphite

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Growth of silicon nanostructures on graphite

Paul Scheier 1, Bjo¨rn Marsen, Manuel Lonfat, Wolf-Dieter Schneider 2,

Klaus Sattler *

Department of Physics and Astronomy, University of Hawaii at Manoa, 2505 Correa Road, Honolulu, HI 96822, USA

Received 23 November 1999; accepted for publication 14 February 2000

Abstract

Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG) Scanning tunneling microscopy (STM ) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2 nm, 5.1±1.2 nm, and 15.4±3 nm diameter Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon Most of the deposited material, with an average diameter of 2 nm, is found to be attached to the monatomic step edges forming the crater rims A simulation of the growth process, i.e deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most

of the Si nanostructures observed in the STM topographs © 2000 Elsevier Science B.V All rights reserved

Keywords: Clusters; Computer simulations; Growth; Scanning tunneling microscopy; Silicon; Sputter deposition

1 Introduction clusters by Honig [14], several experimental

inves-tigations on silicon clusters have been performed [15–30], including a few STM studies [23,26–30] Clusters deposited on well-defined surfaces

Kuk et al [23] deposited Si

10clusters on Au(001)

allow the construction of new materials with novel

and observed a wide variety of different cluster properties [1] The current urge for an ever

images, even though size-selected clusters were decreasing size of components in the

microelec-deposited McComb et al [26 ] observed a site-tronics industry renders this particularly relevant

specific variation in the electronic characteristics for silicon clusters [2] Their electronic and optical

of Si clusters, which were deposited without size properties are especially sensitive to their size and

selection but observed with atomic resolution structure [3–13] Since the earliest study on silicon

Dinh et al [27,28], in the context of an investiga-tion of the optical properties of passivated Si

* Corresponding author Tel.: +1-808-956-8941;

nanostructures, synthesized Si nanocrystals by fax: +1-808-956-7107.

E-mail addresses: paul.scheier@uibk.ac.at (P Scheier), laser ablation and by thermal evaporation in an sattler@hawaii.edu ( K Sattler) Ar buffer gas, and determined the size distribution

1 Permanent address: Institut fu¨r Ionenphysik, Universita¨t of a monolayer of these nanostructures on HOPG Innsbruck, A-6020 Innsbruck, Austria.

with an STM Size-selected Si

30 and Si39 clusters

2 Permanent address: Institut de Physique de la Matie`re

were imaged with a low-temperature STM on Condense´e, Universite´ de Lausanne, CH-1015 Lausanne,

0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V All rights reserved.

PII: S 0 0 39 - 6 0 28 ( 00 ) 0 04 2 6 -X

appearance of the clusters in the images indicated pressure of p<10−6 Pa This chamber was con-soft-landing of the clusters Recently, again in an nected via vacuum locks to an analysis chamber STM study, Marsen and Sattler [30] succeeded in (base pressure p<10−8 Pa) equipped with a

creating fullerene-structured nanowires of silicon Nanoscope II scanning tunneling microscope

by magnetron sputtering on HOPG substrates (STM ) from Digital Instruments For the synthesis The present STM study intends to investigate in of the Si nanostructures, a magnetron sputter more detail the sub-monolayer and monolayer source (MightyMak, Thin Film Products) was growth regimes of Si nanostructures on defect- used In an argon atmosphere of 600 Pa at a poor and defect-rich HOPG surfaces discharge voltage of 600 V and a typical Ar ion

current of 0.2 A, a Si deposition rate of 0.3 nm/s was obtained A quartz crystal micro-balance

2 Experimental mounted at a distance of 10 cm from the Si target

monitored the flux during deposition The cleaved HOPG substrate (7×7 mm2), used to collect the The synthesis of Si nanostructures was

per-formed in a high-vacuum chamber with a base sputtered Si, was mounted in a copper block

Fig 1 (a–c) Room-temperature constant-current topographs of a HOPG surface area at more than 1 ML coverage with silicon clusters Image size: (a) 1.1×1.1 mm2, (b) 44×44 nm2, (c) 10×10 nm2; tunneling parameters: (a) U=1.0 V, It=0.32 nA; (b, c) U=

2.5 V, I

t=0.46 nA (d ) Line-scan along the white line indicated in (c) (e) Size distribution of the silicon nanoclusters determined from the STM images (a–c).

(equipped with heating and cooling facilities) 5 cm contrast to Ref [30], the amount of silicon

depos-ited in the presently shown examples was much

in front of the sputter source A manually operated

shutter was placed between the sputter source and smaller since both the opening of the shutter and

the argon ion current were reduced (<45 s instead the substrate holder during precleaning of the Si

target and served to control the Si arrival fluences of 2 min and 50 mA instead of 200 mA) After

deposition, the sample was transferred in situ into The average size of the Si clusters synthesized by

this technique could be varied by changing the the STM chamber in order to characterize the

deposited silicon nanostructures under stringent sputter parameters, increasing (or decreasing) the

source-to-substrate distance, or a combination of ultra-high-vacuum ( UHV ) conditions All STM

topographs presented in this work were taken with all these parameters [30] In the present

experi-ments, typical exposure times were varied from a Pt/Ir tips on the same sample and were recorded

in constant current mode The bias voltage between few seconds to about a minute, yielding isolated

clusters or cluster films with a thickness of 1– tip and sample is taken with respect to the latter

Tunneling resistances in the range between 100 MV

3 ML (monolayers) on HOPG, respectively In

Fig 2 (a–c) Room-temperature constant-current topographs of a HOPG surface area at about 0.1 ML coverage with silicon clusters Image size: (a) 400× 400 nm2, (b) 94 × 94 nm2, (c) 41 × 41 nm2; tunneling parameters, (a–c) U=1.96 V, It=0.32 nA (d) Section

of the HOPG hexagonal surface lattice showing the angle between armchair and zigzag directions.

Fig 3 (a–g) Room-temperature constant-current topographs of a HOPG surface area with nanopits of various depth at about 0.5 ML coverage with silicon clusters Image size: (a) 350 ×350 nm2, (b) 150×150 nm2, (c) 100×100 nm2; (d) 20×20 nm2; (e) 86×86 nm2; (f ) 100×100 nm2; (g) 40×40 nm2; tunneling parameters: (a) U=−1.5 V, It=0.38 nA; (b) U =1.1 V, It=4.2 nA; (c) U=−1.3 V,

I

t=0.38 nA; (d) U =−1.4 V, It=0.38 nA; (c) U =1.6 V, It=0.26 nA; (c) U =0.73 V, It=0.51 nA; (c) U =1.2 V, It=0.51 nA (h) Constant-current topograph of a small island in the center of (f ) showing the graphite surface lattice with atomic resolution Image size: 3×3 nm2; tunneling parameters: U=1.2 V, It=0.68 nA.

and 6 GV yield identical images Very similar ters Two step edges of the HOPG substrate are

clearly visible in the image due to the dense images have been obtained from other samples

prepared under the same experimental conditions decoration with a chain of clusters Fig 1b and 1c

show a 44×44 nm2 and a 10×10 nm2 area, taken across the left step in the bottom of Fig 1a These images reveal round Si-structures in the size range

3 Results

from 1 to several nanometers A cross section, indicated by a white line in Fig 1c and shown in Fig 1a shows a 1.1×1.1 mm2 area of an HOPG

surface covered with about 3 ML of silicon clus- Fig 1d reveals that the smallest round structures

Fig 3 (continued)

are semi-spherical with a diameter (FWHM ) of thermal evaporation in an Ar buffer gas and

collected on HOPG, where the gathering of the Si about 1 nm Due to the convolution of tip and

object geometries the clusters appear larger as in nanoclusters at step edges as well as their

self-assembly into superclusters has been noted [31] reality To correct for this effect we evaluated the

tip dimensions on the widths of monatomic steps Fig 2a shows an STM topograph of a

400×400 nm2 area of HOPG taken at a lateral

of pure HOPG yielding a tip contribution of

0.3 nm Fig 1f shows the corrected size distribu- distance of several micrometers from the region

shown in Fig 1 Three step edges cross the image tion of about 1000 Si-clusters obtained from an

analysis of Fig 1a -e It follows that all observed from the bottom to the top The two uppermost

layers of graphite are partially folded back on nanostructures fall into three relatively narrow size

ranges The smallest structures have an average their left-hand side, a phenomenon already well

known from earlier STM studies of HOPG [32,33] diameter of 0.6±0.2 nm, containing up to 10 Si

atoms [3–13,29] Larger aggregates exhibit diame- In contrast to the observations made in Fig 1, the

silicon coverage at this new position with a higher ters of 5.1±1.2 nm, and the largest superclusters

have sizes in the range of 15.4±3 nm This obser- density of defects is significantly smaller (about

0.1 ML), and the step edges are less densely decor-vation suggests that the small clusters of 1 nm

diameter constitute building blocks for the larger ated, although the flux of silicon atoms is expected

to be homogeneous over much larger surface areas aggregates These findings confirm similar

observa-tions made in a recent atomic force microscopy In the lower part of the uppermost terrace, an

elongated Si structure is visible A close-up of a (AFM ) study of Si nanocrystals synthesized by

94×94 nm2 area of this region reveals a chain of the Si-step decoration of the upper step edge (see

Fig 3c)

silicon clusters at an angle of 41.3° with respect to

We summarize our main experimental observa-the step edge A combination of armchair and

tions on the growth of Si nanostructures on HOPG zigzag directions in the 2D-graphite hexagonal

as follows The average silicon coverage varies by network yields an angle close to this value, as

a factor of more than 10 between the surface illustrated in Fig 2d We conclude that the

regions of different defect densities, separated by arrangement of the carbon surface atoms in this

only 0.1 mm The diameter of the clusters formed crystallographic direction provides favorable

bind-onto defect-poor, flat surface regions is about ing sites for such a chain-like structure An closer

0.6 nm, while clusters attached to step edges or look at this structure (Fig 2c) reveals that the

defects have diameters of about 2 nm and, occa-segments of this cluster chain have an average

sionally, are found to be fused into rod- or tubelike thickness of 3.1±0.3 nm (see line scan) with

structures In the coverage range between 0.5 and lengths varying from 2.3 to 7.5 nm (uncorrected

5 ML, these small clusters often form superclusters values) In addition, on the two terraces shown in

The density and size of the clusters attached to the Fig 2b and c, uniformly sized (1.1±0.1 nm) Si

step edges forming HOPG nanopits are indepen-clusters are distributed randomly Most of these

dent of the width of their ‘feeding’ terraces The small silicon clusters form distinct, loosely packed

rims of nano-sized graphite islands on HOPG are groups (only 10% of the small clusters have no

practically free of silicon decoration

neighbors)

Fig 3a–f shows constant current images of

sur-face areas containing craters and pits [34] with

4 Simulation

depth down to 10 ML Every step edge of the

descending terraces is decorated with a chain of

In order to rationalize the above observations, silicon clusters The average diameter of all cluster

we simulate the growth process, i.e adsorption, chains in this area is 3.1±1.1 nm (see Fig 2c, line surface diffusion, and clustering of the silicon scan) and thus seem not to depend on the width

atoms on HOPG within a simple two-dimensional

of the terraces limited by the steps This value

model, as sketched in the flow diagram shown corresponds well with the average diameter of the in Fig 4.

cluster chain observed in Fig 2 Almost no silicon In a first step, the topography of the surface is clusters are found on the flat terraces Only in the defined Within a two-dimensional array of upper left-hand corner of Fig 3a is the density of 400×400 pixels, the location of a step or defect is the silicon clusters large enough to cover more assigned a value of 1 (black pixel ), and all other than just the steps Images taken of adjacent positions are set to zero (white pixel ) Fig 5a surface regions in this direction exhibit structures displays a typical example of such a model surface that are identical to those found in Fig 1 corresponding closely with the experimental STM

In Fig 3b, c, f, and g, flat islands with diameters image shown in Fig 3a The number, n, of indivi-between 5 and 20 nm are visible on the larger dual Si atoms adsorbed on the surface is chosen. terraces A high-resolution image of one of these Furthermore, the number of diffusion jumps of a islands is shown in Fig 3h A periodic lattice single atom before desorption is considered (max-identical to that of graphite is observed, which steps), representing the residence time at the allows us to identify these small islands as genuine surface.

graphite nanoflakes We note that we were able to All adsorbed atoms move randomly along the obtain this pattern only on very few HOPG flakes, surface Whenever an atom desorbs or attaches to indicating a shift and/or rotation of these flakes a nucleation site, such as a cluster or defect, it is with respect to the underlying graphite layers replaced by a new one In case of attachment, the Finally, we note that in the STM images shown number of Si atoms at the nucleation site is

increased by one Collision with another adsorbed

in Figs 1–3, the respective line scans clearly reveal

Fig 4 Flow diagram of a two-dimensional growth simulation n is the number of single atoms that are attached to the surface at all

times If an atom desorbs, di ffuses out of the defined surface area or fuses to a cluster or defect, it is replaced by a new incoming

atom n is proportional to the flux of impinging atoms maxsteps is the number of diffusion steps that a single atom can perform before it desorbs from the surface.

atom leads to the formation of the smallest cluster, f displays simulated growth patterns for a surface

exhibiting the characteristic topography of Fig 3a

a dimer The probability for the desorption and

the diffusion of clusters is set to zero Therefore, The total number of adsorbed atoms that form

clusters and decorations of defects is one million once a cluster is formed on a terrace, its migration

to a defect is excluded, and a new nucleation site for all four simulations An atom density of n=2

atoms was used in the case of images Fig 5c and

is created Fig 5b shows the number of atoms

attached to each pixel of a surface area exhibiting d and n=1000 atoms for images Fig 5e and f In

Fig 5c and e, the residence time was short (max-five step edges in terms of a bar diagram The

height of each column represents the number of steps=1) whereas in Fig 5d and f, an infinite

residence time was assumed (maxsteps=2) For atoms at this location at the end of a simulation

For a direct comparison of the simulated results the low atom density, clusters are formed almost

exclusively along the steps and defects ( Fig 5c and with the STM images, semi-spherical clusters were

plotted where the cube of the radius is proportional d ) In contrast, at high atom density, clusters are

also formed on the terraces (Fig 5e and f ) At

to the number of atoms within the cluster Fig 5c–

Fig 5 Simulation of the Si-cluster growth (see text) (a) Model surface with defects (black lines) (b) Bar diagram of the number of accumulated atoms at a few step edges (c, d ) Growth pattern after low-Si-atom density deposition at short (c) and long (d) residence times of the Si-atoms at the surface (e, f ) Growth pattern after high-Si-atom density deposition at short (e) and long (f ) residence times Note the excellent agreement of the results of these simulations with the observed growth patterns shown in Fig 3.

short residence times, clusters are very uniform in flakes has a cluster attached to its edge We

attri-bute this observation to two effects:

size ( Fig 5c and d ), whereas for an infinite

resi-dence time, their size depends strongly on the size 1 A vanishing Schwoebel–Ehrlich barrier on the

step edge of these very small HOPG islands

of the feeding terrace

allowing for interlayer diffusion At a critical island size, such an effect has been invoked to

be responsible for ‘landsliding’ on small Cu

5 Discussion

islands [36,37]

2 Bond weakening of the HOPG nanoflakes

A comparison of the experimental STM image

towards the adsorbed Si atoms due to weak

of Fig 3a with the results of the simulation

dis-coupling of these flakes to the underlying graph-played in Fig 5 clearly suggests that our simulation

ite surface In the case of Pt on HOPG, the captures the essential physics of the growth

pro-perfect stacking of the graphite layers has been cess After adsorption of single silicon atoms on

shown to be important for optimal bonding the HOPG surface, these adatoms move randomly

[38,39]

by thermal diffusion along the surface Collisions

among them lead to the growth of Si clusters Step

edges with unsaturated or dangling bonds

consti-6 Summary and conclusions

tute preferred nucleation sites and exhibit an

effec-tive Schwoebel–Ehrlich [35] barrier for interlayer

Silicon nanoparticles were synthesized using

diffusion of Si adatoms On surface regions with

magnetron sputtering deposition onto cleaved

an increased defect density, we observe a much

HOPG The resulting Si nanostructures were inves-smaller coverage of silicon (see Figs 2 and 3:

tigated with STM On defect-poor, flat regions of 0.1 ML) than on surface regions with a low defect

the HOPG surface, Si clusters with a mean diame-density (see Fig 1: 3 ML) For the simulation

ter of about 0.6 nm and a narrow size distribution shown in Fig 5c (which gives the best agreement

were found On defect-rich surface regions, step with the STM topograph), the residence time of

edge decoration was observed almost exclusively, the adatoms was short In other words, most of

while the terraces were free of attached particles the diffusing Si atoms desorb from the surface

A simple two-dimensional simulation of the Si before they encounter a defect or collide with

cluster growth successfully describes most of the another Si atom and form a cluster on a terrace

experimental observations, e.g the gathering of For Fig 5c, the second parameter used in the

clusters on step edges and the formation of clusters simulation (the density of silicon atoms) was low

and superclusters on the terraces The simulation This parameter can be interpreted as a

combina-leads to the conclusion that the sticking coefficient tion of the flux from the sputter source and the

of the HOPG surface depends on the density of sticking coefficient In order to end up with the

the defects The STM topographs reveal that the same number of 1 million silicon atoms attached

silicon coverage on a defect-rich surface region is

to the surface (either in the form of clusters on

smaller by a factor of~30 than on a defect-poor terraces or decorations of edges and defects), the

region of the same sample In view of the present actual number of initial silicon atoms that hit our

results, magnetron sputtering might provide an model surface was, in the case of Fig 5c, about

interesting alternative route towards the

pro-100 times larger than for Fig 5f This agrees very

duction of Si nanostructures with potential appli-well with the experimental observation that a

cations in future silicon nanotechnology

defect-poor region has about 30 times more silicon

attached than the defect-rich region of the same

sample

Acknowledgements

The areas that exhibit pits and craters also

contain small HOPG islands or flakes that are

practically free of adsorbed silicon particles For P.S gratefully acknowledges an APART grant

from the Austrian Academy of Sciences, and example, Fig 3c reveals that only one out of 20

[20] W.L Wilson, P.F Szajowski, L.E Brus, Science 262 W.D.S thanks the Swiss National Science

(1993) 1242.

Foundation for financial support

[21] C Delerue, M Lannoo, G Allan, E Martin, I Mihal-cescu, J.C Vial, R Romestain, F Muller, A Bsiesy, Phys Rev Lett 75 (1995) 2228.

[22] A.A Shvartsburg, M.F Jarrold, B Liu, Z.-Y Lu, C.-Z Wang, K.-M Ho, Phys Rev Lett 81 (1998) 4616 [23] Y Kuk, M.F Jarrold, P.J Silverman, J.E Bower, W.L.

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