Surface Science Letters

Light emission from a single atom

M. Sakurai a,*, C. Thirstrup b, M. Aono a,c

a National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japanb Vir A/S, Kuldyssen 10, 2630 Taastrup, Denmark

c Department of Precision Science and Technology, Osaka University, Suita, Osaka 565-0871, Japan

Received 18 October 2002; accepted for publication 14 November 2002

Abstract

Scanning tunneling microscope (STM)-induced light emission from a single Si dangling bond on a Si(0 0 1)-(2� 1)–

D surface was studied at room temperature. Switching events of a single Si dangling bond were observed in the light

intensity map and in the topographic image recorded at a bias voltage of )3 V. High spatial resolution of a light in-

tensity map supports that the light emission is due to radiative transition of electrons between the localized surface state

of Si dangling bond and the surface state of the STM tip.

� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Scanning tunneling microscopy; Luminescence; Surface electronic phenomena (work function, surface potential, surface

states, etc.); Silicon; Deuterium

Manipulation of light using nanoscale struc-

tures has been studied actively [1], and the studies

are related to applications such as photonic or

optoelectronic devices. Scanning tunneling mi-

croscope (STM)-induced light detection is one

method employed to study optical properties of

nanoscale structures [2]. Utilizing the fact that theSTM tip is an atomic scale injection source enables

us to measure the optical response of a single atom

and helps understanding the mechanisms of STM-

induced light emission [3]. In a present paper, the

studies have been focused on precise spatial dis-

tribution of the light emission from a single Si

dangling bond at room temperature. Although

light emission from a single molecule has been

studied using near-field optics [4], the present

sample made of Si and deuterium (D) atoms has

the unique character that the area of light emission

is localized to a single Si atom site and that the

position of a single Si dangling bond can be con-

trolled by the bias voltage applied to the STMtunneling gap [5]. Based on experimental results,

we propose a single atom light switch.

All experiments were performed in an ultra-

high vacuum (UHV) chamber with a base pressure

of 1� 10�8 Pa using an STM and electrolytically

sharpened tungsten (W) tips. Exposure of a clean

Si(0 0 1)-(2� 1) surface (antimony-doped, n ¼ 1�1018 cm�3) to atomic D gas at a sample tempera-ture of 600 K produced a Si(0 0 1)-(2� 1)–D surface

structure, similar to the hydrogen (H)-termination

technique [6]. In the present experiment, D atoms

*Corresponding author.

E-mail address: sakurai.makoto@nims.go.jp (M. Sakurai).

0039-6028/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0039-6028(02)02654-7

Surface Science 526 (2003) L123–L126

www.elsevier.com/locate/susc

were used rather than H atoms for the surface

termination, because the STM-induced desorption

yield of D is much lower than the corresponding

desorption yield of H [7]. After creating a few Si

single dangling bonds by the extraction of D atoms

using the STM tip [8], the light demitted from thetunneling gap was collected by an optical fiber

bunch, and it was guided to a detecting plane of a

water-cooled photon multiplier tube [9]. The de-

tectable range of light in the present system is 350–

850 nm.

Fig. 1(a) and (b) show occupied-state and un-

occupied-state STM images of a Si(0 0 1)-(2� 1)–

D surface, respectively. Bright spots on the terracein Fig. 1(a) correspond to single Si dangling bonds.

In Fig. 1(b), the dark ring surrounding the bright

spot of a single dangling bond is formed by the

change of potential. The electric charge occupied

on a surface state of a single dangling bond [10] is

screened by the holes induced by the positive

sample bias voltage. Then the decrease of electric

charge density around a single dangling bond leads

to the dark ring in the unoccupied-state image.

The ring pattern in the unoccupied-state image can

be used as judgment on a single or double dangling

bond. The bright spots marked A, B, C, and D in

Fig. 1(a) correspond to single dangling bonds. Fig.

1(c) and (d) are a light intensity map and a topo-graphic STM image of the same area, respectively.

The images were recorded simultaneously at a

sample bias voltage (Vs) of )3 V, a tunneling cur-

rent (It) of 3 nA, and a scanning velocity (vt) of 9nm/s. In the light intensity map, light is emitted

when the tip is located above Si dangling bond

sites. The light intensity on dangling bond sites is

100–150 counts per second (cps), which corre-sponds to the quantum efficiency of the order of

10�6 photons/electron. The average light intensity

on D-terminated areas is 3–5 cps, which is com-

parable to the dark count level of the system.

The contrast formation in the light intensity

map is caused by the difference of the surface states

below the tip apex from following reasons. Bulk Si

materials emit few photons due to its indirect bandgap. Tip-induced surface plasmon gives small

contribution to the light emission, because plasma

resonance energies of Si and tungsten materials

formed the tunneling junction are not in the visible

spectral range. On Si dangling bond sites, surface

states of the Si dangling bond are positioned near

the Fermi level. On D-terminated areas, the sur-

face states are lost by the bond formation. Theninelastic tunneling of electrons from the occupied

surface state of Si dangling bond near the Fermi

level to an unoccupied surface state of the tip leads

to the visible light emission.

Fig. 2(a) shows the magnified topographic im-

age of the single Si dangling bond marked as C in

Fig. 1(d), suggesting that the Si dangling bond

switches sites during the tip scanning. On the lineas marked by the white arrows, the Si dangling

bond is located on the right hand side of the Si

dimer. On the line as marked by the gray arrows,

the dangling bond is on the left hand side of the Si

dimer. The scanning condition of Vs ¼ �3 V is in

the range that an STM-induced switch of a single

Si dangling bond occurs frequently (�3:2 <Vs < �2:2 V) [5].

Fig. 2(b) shows the corresponding light inten-

sity map of a single Si dangling bond. The light

A

B

CD

(a) (b)

(c) (d)

6 nm

A

B

C

D

A

B

C

D

A

B

CD

Fig. 1. Topographic (a) occupied-state and (b) unoccupied-

state STM images of a Si(0 0 1)-(2� 1)–D surface with a few

single Si dangling bonds. (c) Light intensity map and (d) to-

pographic image of the same area recorded simultaneously at

Vs ¼ �3 V, It ¼ 3 nA, and vt ¼ 9 nm/s.

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intensity map exhibits switching of the light in-tensity as similar to the switching of a single Si

dangling bond in the topographic image. Fig. 2(c)

depicts cross-sectional line profiles of the light in-

tensity map. In the second, third and seventh line

profiles, the protrusion of light intensity is posi-

tioned on the right hand side, whilst in the fifth

and sixth profiles the protrusion is positioned onthe left hand side. The region of the light emission

changes similarly to the motion of the Si dangling

bond. In the fourth profile, light is not emitted,

because the switching event occurred when the tip

was located near the right hand side of the dimer

where the Si dangling bond occupies. The rela-

tionship among the tip, the single Si dangling

bond, and the light emission is schematically il-lustrated in Fig. 2(d). The lateral extension of the

light emitting area is the same as that of the wave

function of the single Si dangling bond [11], sug-

gesting that the event of the light emission oc-

curred by the transition from the localized surface

state of the single Si dangling bond. It should be

noticed that the switching event is caused by the

STM tip, whose electric field reduces the energybarrier between the two sites of a dimer [5]. Then

the light emission including the switching can be

controlled by the precise position of the Si dan-

gling bond to the tip apex under suitable bias

voltage condition.

Fig. 3(a) and (b) are occupied-state and unoc-

cupied-state STM images of a Si(0 0 1)-(2� 1)–D

surface with a few Si dangling bonds, respectively.The Si dangling bonds marked as A and B in the

figures are double Si dangling bonds, whose pH

state in the unoccupied-state image exhibits a node

plane between the dimer atoms [12]. Fig. 3(c) and

(d) shows topographic STM image and light

1 nm

(a) (c)

(b)

(d)

ON OFF

0.8 nm

Si dangling bond D atom

STM tip

Si dimer

Light

Ligh

tin

tens

ity[a

rb. u

nits

]

Fig. 2. (a) Focused topographic image around the single Si

dangling bond marked as C in Fig. 1(d). The Si dangling bond

switches sites during the line scan. (b) Corresponding focused

light intensity map. (c) Cross-section line profile of light in-

tensity along each horizontal line in (b). (d) Schematic illus-

tration of a single atom light switch using the STM tip apex and

one Si dimer with one Si dangling bond.

(a) (b) (c) (d)

A

B

A

B

A

B

A

B

3 nm

Fig. 3. Topographic (a) occupied-state and (b) unoccupied-state images of double Si dangling bonds as marked A, B. (c) Topographic

STM image and (d) light intensity map of the same area recorded at Vs ¼ �3 V, It ¼ 3 nA, and vt ¼ 9 nm/s.

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intensity map of the same area recorded at Vs ¼ �3

V, It ¼ 3 nA, and vt ¼ 9 nm/s. The cross-sectional

line profile of the double Si dangling bonds in the

light intensity map has the similar protrusion tothat of the p state of the Si dimer in the topo-

graphic image, supporting that the light is created

by radiative transition of electrons from the oc-

cupied p state of double Si dangling.

Fig. 4(a) and (b) are occupied-state and unoc-

cupied-state STM images of Si(0 0 1)-(2� 1)–D

surface with a few Si dangling bonds. Fig. 4(c) is

the light intensity map of the same area recordedat Vs ¼ þ3 V, It ¼ 0:4 nA, and vt ¼ 9 nm/s. In the

case of the positive Vs, the switching event of a

single Si dangling bond was rarely observed. The

lateral extension of the light emission is localized

to the Si dangling bond sites, indicating that the

light is created by the radiative transition of elec-

trons to the localized surface state of the Si dan-

gling bond.Since the light is emitted from a single Si atom

below the tip apex and since the switching of a

single Si dangling bond can be controlled by the

applied bias voltage, a simple system made of the

STM tip and one Si dangling bond can be used as

a single atom light switch. The idea is schemati-

cally illustrated in Fig. 2(d). When the tip is lo-

cated above a single Si dangling bond, light is �ON�(left panel in Fig. 2(d)). If we apply suitable neg-

ative Vs between the tip and the single Si dangling

bond, the Si dangling bond switches to the other

site of the dimer and switches off the light (right

panel in Fig. 2(d)).

In summary, intensity maps of light emission

from a single or double dangling bond on D-ter-

minated Si surface have the high spatial resolution.

The spatial localization of the light emission on the

dangling bond sites at both polarities of Vs sup-

ports the mechanism of the light emission by theradiative transition between the surface states

across the tunneling gap. Present results based on

a single atom light emission suggest one possibility

of forthcoming single atom devices.

Acknowledgements

This research was supported by Atomic-Scale

Sciengineering Research in RIKEN (The Institute

of Physical and Chemical Research).

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(a) (c)(b)

A

B

C

D

A

B

C

D

A

B

C

D

3 nm

Fig. 4. Topographic (a) occupied-state and (b) unoccupied-

state STM images of a Si(0 0 1)-(2� 1)–D surface. (c) Light

intensity map at Vs ¼ þ3 V, It ¼ 0:4 nA, and vt ¼ 9 nm/s on the

area as shown in (a).

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