Surface Science 573 (2004) L363–L368

www.elsevier.com/locate/susc

Surface Science Letters

Imprinting Br-atoms at Si(1 1 1) from a SAMof CH3Br(ad), with pattern retention

Sergey Dobrin, Xuekun Lu, Fedor Y. Naumkin,John C. Polanyi *, Jody (S.Y.) Yang

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6

Received 26 August 2004; accepted for publication 21 September 2004

Available online 4 November 2004

Abstract

The formation of molecular-scale patterns at surfaces as a self-assembled monolayer (SAM) demands adsorbate

mobility, whereas devices require stability. We describe a method of photo- or electron-imprinting a SAM of

CH3Br(ad) as covalently-bound Br–Si(s) at Si(1 1 1)7 · 7, with pattern-retention. This imprinting process, it is pro-

posed, involves charge-transfer to the adsorbate, followed by downward recoil of Br� to give chemical attachment

of Br at the reactive Si-atom beneath the parent physisorbed methyl bromide. The electron may subsequently be

returned to the substrate along with the excess energy, accounting for the localised Br-imprint.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Scanning tunneling microscopy; Self-assembly; Bromine; Surface chemical reaction; Photochemistry; Silicon; Chemisorp-

tion; Physical adsorption

The present study lies at the juncture betweenthe fundamental science of molecular reaction

dynamics, studied by scanning tunneling micros-

copy (STM) [1–4], and the applied science of

nano-scale engineering [5–7]. The basic question

is the extent of localisation of a photoinduced or

0039-6028/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2004.09.048

* Corresponding author. Tel.: +1 416 9783580; fax: +1 416

9787580.

E-mail address: jpolanyi@chem.utoronto.ca (J.C. Polanyi).

electron-induced atom-transfer reaction at the sur-face. The reagent is a loosely-bound physisorbed

molecule at 50K, CH3Br(ad)/Si(1 1 1)7 · 7, and

the product a strongly covalently-bound Br–Si(s)

stable at 200 �C, with the methyl radical presumed

to escape into the gas. The application is to the ro-

bust photo- or electron-imprinting of previously la-

bile patterned self-assembled monolayers (SAM�s).Nanofabrication, to be practical, must be per-

formed by large-scale molecular self-assembly

ed.

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(SA) [8]. Self-assembly requires molecular mobil-

ity. But a device fabricated by SA must be stable,

preferably strongly covalently-bound to the sur-

face [9]. The transition from a labile SA pattern

to a stable one can be induced through chemicalattachment. The problem is to retain the SA pat-

tern as a chemically-attached pattern.

This dictates the choice of molecule for SA. In

the language of reaction dynamics, what is re-

quired is a physisorbed molecule that undergoes

localised reaction to form a covalent bond with

the surface. This should be a reaction that can be

induced at a selected time following SA, which isoften slow [8].

We report an experimental study of the SA of

CH3Br(ad) physisorbed on Si(1 1 1)7 · 7 at 50K,

and its subsequent photoreaction or electron-in-

duced reaction. The SA pattern is revealed by

STM to be a limitless array of CH3Br(ad) circles,

with twelve molecules per circle. Following SA,

reaction is induced by pulsed ultraviolet laser radi-ation (193nm) to yield a similar pattern of cova-

lently-bound bromine atoms. Since physisorption

differs from chemisorption, some shift in configu-

ration must occur.

With the polarizable Br-end of the �parent�CH3Br(ad) initially pointing down (see below),

the �daughter� Br formed by reaction will be direc-

ted toward a Si-atom beneath. STM confirms thatdirect covalent attachment occurs. A noteworthy

finding is the lack of degradation of the pattern

in going from SA to chemical attachment, despite

the energy available to the recoiling Br. We present

a model in which reaction is triggered by charge-

transfer of an electron [10–13] to the physisorbed

CH3Br(ad) SAM, resulting thereafter in collision

between recoiling Br� and the underlying surfacewith transfer of the e� to the silicon, and capture

of Br as Br–Si(s) to give localised reaction-

product.

Low-temperature STM was performed in an

ultrahigh vacuum chamber. The experiments were

performed with an Omicron Variable Temperature

STM system with base pressure <1 · 10�10mbar.

The STM was operated in constant current(0.2nA) mode with the tunneling biases from 1.5

to 2.5V at the sample. Clean Si(1 1 1)7 · 7 sur-

faces (B-doped, 0.0493 � 0.0533 ohms Æ cm) were

prepared by heating the sample to �1500K for

100s, at which the chamber pressure was kept un-

der 2 · 10�10mbar. Sample cooling was achieved

by pumping liquid He through a continuous-flow

cryostat. The Si(1 1 1) sample was cooled to�50K and CH3Br (Sigma Aldrich) was dosed by

backfilling the STM chamber to a pressure of

2.5 · 10�9mbar. The chamber was evacuated after

CH3Br adsorption, to ensure photochemistry only

at the surface-adsorbate interface. Ultraviolet light

from an excimer laser (Lambda physik) operated

at 193nm (ArF), at a pulse repetition rate of

10Hz gave photoreaction. The incident angle ofthe laser was approximately 45� from the surface

normal. The fluence at the sample was varied from

40 down to 1.5 mJ/(cm2 pulse) by wire mesh.

In Fig. 1(A) a typical atomically-resolved image

of the Si(1 1 1)7 · 7 surface is shown, with a unit

cell. Each unit cell has 12 Si-adatoms; 6 middle

and 6 corner [14]. At 50K adsorbed methyl bro-

mide molecules appear as bright protrusions cen-tered over the middle adatoms.

Middle adatoms in six adjacent half-unit-cells

form circles of twelve adatoms. Since these com-

prise half the adatoms, we deposited just under

50% coverage of methyl bromide to obtain the

ring-pattern of Fig. 1(B). These bright rings ex-

tended without limit. Each ring consisted of 12

CH3Br(ad) (close-up, Fig. 1(C)).Similar patterns of physisorbed molecules have

been reported for benzene (not previously for

CH3Br) on Si(1 1 1) at 78K [15]. In both cases

the bright features were observed only at low tem-

perature. The preference for physisorption on mid-

dle adatoms may be attributed to the fact that

these are more electrophilic [16,17].

In the case of CH3Br, studied here, irradiationwas found to cause reaction. Irradiation using a

photon flux of 4.0 · 1018cm�2 of 193nm excimer

radiation caused 70% of the adsorbate at 50K to

alter from the bright STM image characteristic

of physisorption to darker features at the same

site, indicative of reaction. The cross-section for

photoreaction (fractional conversion, 0.70, di-

vided by the photon flux) is �1 · 10�19cm2. Sin-gle-photon photoreaction was confirmed by the

linear intensity-dependence of rate for a 27·change in intensity. The calculated temperature in-

Fig. 1. (A) STM image of the clean Si(1 1 1)7 · 7 surface at

50K. A 7 · 7 unit cell is indicated. Vsurface = 1.5V, curren-

t = 0.2nA, �20nm · 20nm. (B) STM image of physisorbed

CH3Br(ad) on the 50K Si(1 1 1)7 · 7 surface at a coverage of

0.41 monolayer. Physisorbed molecules appear as protrusions

over the middle adatoms. Vsurface = 1.5V, current = 0.2nA,

�20nm · 20nm. (C) Zoomed-in STM image of a single ring of

physisorbed CH3Br on Si(1 1 1) surface (indicated by the dotted

circle), as in (B) but �30 A · 30A. (D) Chemisorbed Br on

Si(1 1 1) surface after photolysis of (three successive applica-

tions of) physisorbed CH3Br(ad) at 50K. Br (beneath dotted

circle) appears as depressions on the middle adatoms.

Vsurface = 1.5V current = 0.2nA, �30 A · 30A. (E) STM image

of chemisorbed Br imprints on the middle adatoms (indicated

by a dotted circle) as in (D) but with Vsurface = 2.5V. (F) STM

image of chemisorbed Br on the middle adatoms (dotted-in)

obtained by scanning (a single application of) physisorbed

CH3Br(ad) at 2.5V (scans from lower left to upper right);

Vsurface = 2.5V, current = 0.2nA, �30 A · 30A.

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crease of 5K for a total time of 10�4 s per full irra-

diation cycle of 600 laser pulses (illumination for600 · 10ns/pulse = 10�5 s, implies heating for

�10�4 s) would not result in significant thermal

reaction. Photo-desorption of the physisorbed

molecule was also observed, with a cross-section

of �3 · 10�20cm2.

On average a single exposure yielded eight Br–

Si per circle of twelve adatoms. Fig. 1(D) and

(E) were obtained from three successive adsorp-

tions, followed in each case by a total of one min-

ute of photolysis at 10Hz (i.e. �three cycles�). Whenthe bias voltage was increased from 1.5V to 2.5V

the dark ring-pattern of reaction-product in Fig.

1(D) became the bright ring-pattern of Fig. 1(E);

this �lighting-up� of atomic features with increased

surface bias-voltage is characteristic of atomic bro-

mine covalently bound to silicon at �40K [18].

Further proof that the physisorbed adsorbate

had photoreacted to produce strongly-bound Br–Si(s) was obtained by heating the pattern shown

in Fig. 1(D) and (E) to approximately 200 �C for

70s. There was no observable change, as expected

for covalent binding.

Intact CH3Br molecules would not covalently

bind to silicon. However there is extensive evi-

dence that CH3Br(ad) undergoes photoreaction

to brominate a reactive substrate [10–12,19,20].All evidence points, therefore, to Br–Si(s) as the

product of photoreaction. Since the reaction prod-

uct forms at the Si-atom beneath the parent

CH3Br(ad), we have localised photoreaction.

The localised nature of the bromination was

also demonstrated for electron-induced reaction

in which the adsorbate-covered surface was

scanned at 2.5V and the CH3Br(ad) was foundto react to form a ring pattern as for photoreaction

(Fig. 1(F)); the ring was partial since this pattern

was obtained from single scan. Scanning at in-

creased voltage resulted in some migration of

physisorbed CH3Br(ad), giving rise to the ob-

served streaks.

Facile electron-induced reaction is to be ex-

pected on the basis of the charge-transfer mecha-nism proposed for photoreaction of organic

halides at reactive surfaces [10–12,19,20]. The large

cross-section for photoinduced reaction (here, as

in earlier work) suggests UV absorption by the

substrate, rather than by the adsorbate. The nov-

elty, in the present STM study, is the localised nat-

ure of the induced surface reaction.

In Fig. 2 we give the results of an ab inito calcu-lation using Gaussian 98 in the MP2/3-21G*

approximation [21] for the attachment energy of

CH3Br(ad) physisorbed at a Si cluster. With the

Fig. 2. CH3Br physisorption equilibrium geometry (C–Br at

approximately 60� to surface normal). The cluster is Si13H18

distributed in three layers (a = adatom, r = rest atom).

Fig. 3. Simple DFT ab initio model of the charge transfer (CT)

reaction with colinear C—Br—Si: (1) CH3Br(ad) + e� gives

CH3 Æ Br�, (2) CH3 Æ Br

� gives Br� Æ SiH3, and (3) Br� Æ SiH3

gives Br–SiH3 + e�. The dots indicate repulsion. Repulsion in

step 2 was calculated separately for CH3 Æ Br� and Br� Æ SiH3.

VEA = vertical electron affinity; Ea = activation energy;

�DH = heat of reaction.

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Br� atom pointing down the binding energy was

0.39eV. With methyl down the binding energy

was approximately 0.15eV; this configurationwas however unstable with respect to that with

Br� down. (For comparison, the experimental va-

lue for physisorbed benzene is 0.4eV [15].)

Charge-transfer (CT) reaction in the preferred

CH3Br(ad) configuration preferentially results in

the downward recoil of Br� from dissociating

CH3Br�. The Br� recoiling along C–Br would im-

pact the surface between a Si-adatom with a dan-gling bond in Fig. 2, and a second relatively

unreactive Si.

A question arises as to why the Br� striking the

underlying Si-adatom at an angle to the normal

does not migrate to an adjacent adatom. The ex-

cess energy of Br� has been estimated as �2eV

for the case of CH3Br(ad)/Pt(1 1 1) [13]. (This en-

ergy includes a term for the attraction of the Br�

by its image-charge in the surface. Since the semi-

conductor surface has been rendered partially

metallic by illumination or tunneling current, a

similar term may apply.) Some of this translational

energy in the Br� (precise energy unknown) will be

absorbed by the surface through translation-to-

vibration energy-transfer. A more effective mode

of energy-dissipation, however, could be the cool-ing of the hot electron when it is returned to the

surface by the Br�.

The initial step of CT reaction requires an elec-

tron of energy equal to the vertical electron affinity

(VEA) of CH3Br, for which an experimental value

exists of 2.4eV [22], and calculated values of 2.11V

[22] and 2.6V [23]. Ref. [13] gives, a comparable

threshold energy for the charge-transfer photore-

action of CH3Br(ad) at GaAs of �2.5eV [12].

Our current–voltage curves, with the STM tipplaced over a CH3Br(ad), exhibited a �negativedifferential resistance� [24–26] at approximately

2.5eV. This was also found to be the threshold

voltage for electron-induced reaction, as indicated

by the appearance of Br–Si features at the sur-

face.

In the DFT approximation [21] used to generate

the potential curves of Fig. 3 (namely B3LYP/6311G*) the VEA was 1.9eV. The CT reaction,

simply modeled as H3CBr + SiH3 at a fixed end-

to-end C—Si separation of 5.3 A appropriate to

physisorption, is indicated by the pathway 1—

2—3 as Br moves from C to Si. Localised reaction

would be expected if the �hot� electron lost its en-

ergy to the surface during the �10�13 s that Br�

spent at the turning point of its vibration againstSiH3. Schmuttenmaer et al. [27] have measured

the relaxation time of �2eV electrons at a semi-

conductor surface as significantly less than

10�13 s. Cooling by this reverse-CT could therefore

be the reason that the recoiling Br� is captured

as Br–Si at the first reactive Si-adatom that it

encounters, with resultant localisation of its

imprint.

Fig. 4. Schematic representation of (A) physisorption of CH3Br on Si(1 1 1) surface with Br pointing down, and (B) chemisorbed Br

on middle adatom positions, after photolysis or electron-impact at 50K.

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The present study of the photo- or electron-in-

duced reaction of a SAM of methyl bromide at

silicon should assist in the choice of molecular

�inks� for imprinting nano-patterns [28]. Recentdiscussion of surface-reaction distinguishes �par-ent-mediated� from �daughter-mediated� dynamics

(parent and daughter, here, being methyl bromide

and atomic bromine) [18,29]. We suggest that

methyl bromide lies in the second category, in

which the adsorbate (the ink) in its initial physi-

sorbed state is directly attached to the surface at

the atom to be covalently imprinted, namely Br.This �direct� (daughter-mediated) mode of reac-

tion, illustrated graphically in Fig. 4, appears

well-suited to the chemical imprinting of SAM

nano-patterns, since it localises reaction to the

point of patterned physisorbed attachment.

Acknowledgments

The authors thank the Natural Sciences and

Engineering Research Council of Canada

(NSERC), Photonics Research Ontario (PRO) an

Ontario Centre of Excellence, and the Canadian

Institute for Photonic Innovation (CIPI), for their

support of this work.

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