扫描隧道显微镜诱导的发光 - 中国科学院物理研究所

国家自然科学基金委员会 

数理学部实验物理讲习班 

Introduction to STM induced Luminescence 

扫描隧道显微镜诱导发光研究简介 

Zhen-Chao Dong (董振超) 

University of Science and Technology of China 

(中国科学技术大学) 

Email: zcdong@ustc.edu.cn 

March 29, 2007 

e – 

+ 

_ 

hv

Why? 

Definition 

Motivation 

What? 

Outline 



What have been done? 

What to do next? 

How? 

Experimental techniques 

Theoretical calculations

1. Brief Introduction to the Background 

• Definition 

• Motivations 

• Luminescence from metal-oxide-metal (MOM) tunnel junctions 

• Early studies of photon emission by STM 

2. Experimental Techniques 

• Photon collection systems 

• Photon detection systems 

• Modes of measurements 

3. Status of STM Induced Luminescence 

• Metals 

• Semiconductors 

• Molecules and Nanostructures 

• Theoretical models 

4. Summary 

Contents 


数理学部实验物理讲习班



1. Brief Introduction 

Scanning Tunneling Microscope 

(STM) Induced Luminescence: 

Definition? 

Motivations? 

Early History?

Definition: STM Induced Luminescence (STML) 

Beyond imaging and manipulation 



e – 

+ 

_ 

e 

hv 

I = I elastic + I inelastic 

Elastic Tunneling Process: 

Conventional STM phenomena 

• Topography 

• Spectroscopy 

e.g., transport: I-V, dI/dV 

barrier height: I-s 

(Inelastic contributions exist!) 

Inelastic Process: 

Vibrational excitation: d 2 I/dV 2 (IETS) 

Tip-induced plasmon 

} 

Radiative decay! 

Electronic excitation STML or STL 

Tunneling electrons: 

Highly localized 

excitation source for photon emission! 

Tunneling electron excited luminescence!

Motivations 

Fundamental Interest: 



• Photons offer various channels for probing tunnel junctions (inelastic process) 

(intensity, spectra, emission pattern, polarization, decay kinetics) 

• Coupling of electrons, excitons, plasmons, and photons in the nano-cavity 

• Fluorescence (exciton) decay kinetics & energy transfer at the nanoscale 

• Quantum tuning of photonic states for nano-objects in a nano-environment 

• Charge transport dynamics at the interface & through nanostructures 

• “Color” STM: chemical identification with high spatial resolution 

• New quantum phenomena (strong field enhancements and NF excitation) 

Practical Interest: 

• Interface parameter control for organic electronics (e.g., OLED, η↑) 

• Spectral imaging of single defects or impurities in electronic devices 

• Radiative decay engineering for bio-molecular (DNA) mapping 

• Nanoscale light source for optical spectroscopy and imaging 

• Single-molecule electroluminescence and single-photon sources

例子: 

I ( V 

) 

STM具有极高的空间分辨力,但缺乏化学分析能力:根源 



∝ 

∫ 

E 

E 

v 

ρ ( r , E ) = 

f 

f 

+ eV 

∑ 

i 

v 

ρ ( r , E ) dE 

v 

ψ ( r ) 

i 

2 

δ ( E 

Atomic Imaging & Manipulation 

Si(111)-7x7 

z-direction: 0.01 nm 

x,y-direction: 0.1 nm 

− 

E 

i 

) 

单原子操纵 

“量子海市蜃楼” 

• STM原子分辨率起源:针尖具有类p z 或d z 2电子态、且结间距很小! 

• 隧穿事件几乎由费米面上的自由电子决定,而且涉及多种态; 

• 隧穿电流涉及样品和探针,是二者电子态密度的卷积。 

探针敏感性 

成象不确定性 ! 

IBM 

D. Eigler



提高STM化学分析能力的途径? 

与谱学技术相结合! 

扫描隧道谱荧光发射光谱 

王兵、裘晓辉等 

专题报告

提高STM化学分析能力的途径一:STM + STS 



空间分辨 

(X,Y) 

扫描隧道谱: 能级结构信息 

空间分辨 + “Z”-分辨 

(X,Y) + I-z 

(X,Y) + I-V 

(X,Y) + dI/dV 

(X,Y) + d 2 I/dV 2 

空间分辨 + 能量分辨 

功函数 

电子态 

特定电子态 

特定振动态 } 

非弹性隧穿谱 

(IETS技术) 

“锁相”技术

提高STM化学分析能力的途径二:STM + FS 



e – 

h + 

STM诱导发光 

ET 

HT 

+ 

_ 

hv 

e–h 

recombination 

原子分辨 

STM 

+ 

单光子计数 

Optical Detectors 

空间、能量、时间 

高分辨 

隧道结 

电荷输运、光学跃迁、 

能量转化动力学 

高分辨 

形貌 

+ 

发光光谱 

高分辨化学标识 

辐射衰变工程(RDE) 

纳米元件、生物分子识别?

e – 

h + 

表征检测工具 



分子尺度调控研究的多功能复合技术 

光学技术扫描探针技术伏安曲线测量 

ET 

HT 

+ 

_ 

e–h 

recombination 

“三合一”技术 

hv 

原子分辨 

STM 

+ 

单光子计数 

Optical Detectors 

空间、能量、时间 

高分辨 

STM诱导发光技术 

空间分辨 

(X,Y) 

“锁相”技术 

“二合一”技术 

空间分辨 + “Z”-分辨 

(X,Y) + “Z” 

(X,Y) + I-V 

(X,Y) + dI/dV 

(X,Y) + d 2 I/dV 2 

空间分辨 + 能量分辨



Microscopy 

in the space domain 

单分子研究 

Single-Molecule Studies 

Spectroscopy 

in the frequency domain 

SNOM: high time resolution (朱星教授) 

Fluorescence decay 

in the time domain 

空间能量时间 

STML: 

imaging and I-V measurements with high spatial resolution 

+ spectra & lifetime



First report of luminescence 

excited by tunneling electrons: 

When? 

Who? 

What system? 

Early STML studies?

Luminescence from metal-oxide-metal (MOM) tunnel junctions 



J. Lambe and S. L. McCarthy, Phys..Rev. Lett. 37 (923) 1976. 

Explanation: 

Radiative decay of plasmons! 

Inelastic tunneling excitation (IET) 

of 

optically coupled 

surface plasmon modes 

V 

Emission feature: 

hv 

Al 

Al2O3 Au or Ag 

CaF 2 

• Broad and continuous; 

• Quantum cutoff: hν ≤ eV; 

• Bias dependent color: 

red (2V) →yellow →blue 

• Visible by naked eye 

(dark room, η= 10-5 –10-4 )

Related Elementary Excitations: Concepts 



Plasmons(等离激元): 

Plasma oscillation― collective motion of the entire electron gas! 

Charge density oscillation: the oscillation quantum of charge density wave. 

Surface Plasmons(表面等离激元): 

Wave-like disturbance of the charge density confined to the surface, 

coherent superposition of electron-hole pairs generated by surface excitons 

Surface Plaritons(表面极化激元或表面极化声子): 

Coupled modes of the surface lattice + electromagnetic field system, 

very long wavelength elementary excitation of the surface coupled with the ambient field 

Surface Plasmon Plaritons(表面等离极化激元): → Surface Plasmons 

Coupled modes of the radiation field and the surface plasmon excitation, 

Surface Plasmons + Surface Plaritons (MOM) 

表面波的特征 : 

电场强度Ez 在界面两侧随距离的增加按指数关系 

快速衰减,在 z=0 处极大,故束缚于界面(表 

面)上,不能检测到远场光。 

A. Zangwill, Physics at Surfaces, 1988.

Oscillating at the plasma frequency 

Plasma oscillation, 

or, charge density oscillation 

Plasmons (等离激元) 



Displacement of the entire electron gas, 

as a whole, by a distance d 

relative to the fix positive background of ions! 

ε(ω) = 1−ω2 p / ω2 

ωτ>> 1 

Condition for the onset of 

propagation of radiation: ω = ω p (ε=0) 

If ε>0 (ω>ω p ), E → oscillatory → propagating 

If ε

Surface Plasmons(SPs, 表面等离激元) 



Δ•Ε = 0 真空-金属界面处 

ε 2 =1 

ε 1 = −1 ω s = ω p /√2 

ω 

ω 

ω p 

ω 

p 

1+ ε 

p 

2 

2 

ε(ω) = 1-ω2 p / ω2 

ω = c ⋅ k 

~ k 

p 

x 

ο −1 

~ 0. 

3 A 

0 

The dispersion of the SPs 

k x 

表面等离激元(表面电磁波) 

存在的条件: 

ε 1 = −|A| ε 2 

界面两侧的介电常数异号! 

光滑结表面: 

k ‖ ≤k r +ω/c 

k ‖ >ω/c 

波矢(动量)不守恒!不能发光! 

粗糙度破坏平移对称性 

波矢匹配!发光!

E F −eV 

MOM 隧道结发光机制 



IET 

Metal Barrier Metal 

隧道结发光是个两步过程: 

HED 

E F 

1、IET electrons excite SPs; 

2、SPs couple with the radiation field 

→ photon emission! 

Optical coupling? → Roughness! 

Radiative decay of plasmons! 

IET: Inelastic Tunneling 

HED: Hot electron decay 

Roughness (粗糙度的引入方式): 

• Intrinsic sample surface roughness 

• Grating structure on the surface 

• Prism coupler via ATR 

• Nanoparticles on the surface 

• Proximity of a sharp tip to the surface 

+ Interband transition of semiconductors



Young et al. (1972) 

FE electrons 

↓ 

hν, sec. e− ↓ 

topography 

(NIST) 

R. Berndt, Wittenberg 2004 

of Junction Luminescence 

Lambe & McCarthy (1976) 

MOM junction, radiative decay of plasmons, IET 

Gimzewski et al. (1988) 

Isochromat Spectra 

↓ 

hν, local IPES 

(IBM Zurich)

First report on STM induced luminescence! 



Photon-emission Experiments with the Scanning Tunneling Microscope 

J. K. Gimzewski, B. Reihl, J. H. Coombs and R. R. Schlittler 

IBM Research Division, Zurich Research Laboratory 

Z. Phys. B72 (1988) 497; J. Microscopy-Oxford 152 (1988) 325. 

Sample 

STM tip 

Detector 

Sample: 

Polycrystalline Ta, Si(111); Ag films 

Mode of measurements: 

Isochromatic spectra with photon energies at 9.5 eV 

Results: 

Spectra reveal electronic structure information 

comparable to inverse photoemission spectroscopy! 

Since 1988, extensive research on the surfaces of 

metals, semiconductors, and adsorbed molecules has been going on! 

Europe, Japan, USA… (30-40 groups)

2. Experimental Techniques 



• Photon collection 

• Photon detection 

• Modes of measurements 

Why limited number of research groups 

around the world on STML? 

Bias 

Adsorbate 

Reference 

current 

Tip 

Spectroscopy 

Radiated 

power 

Sample 

Spectrometer 

S. Ushioda



STM-Induced Light Emission 

Requiring: 

good well-defined sample 

‘good’ tip! 

efficient light collector 

highly sensitive photon detector 

Success!



Light Collection System 

Target: to achieve a large solid angle coverage 

Three types according to optical parts: 

Mirror system: Ellipsoidal/parabolic 

Fiber system 

Lens system 

Two types according to collection modes: 

Far field collection 

Near field collection

UHV Photon STM: Ellipsoidal Mirror Design 



Advantage: 

Large solid angle coverage; 48-61% 

Free of chromatic aberration 

Disadvantage: 

Difficult to make and use; 

Difficult to align and focus 

R. Berndt, R. R. Schlittler, J.K. Gimzewski, J. Vac. Sci. Technolog. B9 (1991) 573. (IBM Zurich) 

Y. Suzuki, H. Minoda, N. Yamamoto, Surf. Sci. 438 (1999) 297. (TIT, Japan)

Advantage: 

Eclipsing of luminescence by STM 

largely minimized → 

UHV Photon STM: Parabolic Mirror Design 



M. J. Romero, et al., Nanolett. released 2006-11-24 

(Natl Renewable Energy Lab, USA) 

High collection efficiency: >10% 

1. N. Nilius, et al., Prog. Surf. Sci. 67 (2001) 

99; Chem. Phys. Lett. 413 (2005) 10. 

2. S. Egusa, et al., Appl. Phys. Lett. 

84 (2004) 1257. 

Disadvantage: 

Difficult to make, align and focus

UHV Photon STM: Fiber Bundle Design 



Single Fiber (Tohoku Univ) 

Φ=600μm, d=1mm, NA=0.2 → 

Solid angle coverage: 0.13 sr, 

Hemisphere collection efficiency: 2% 

T. Tsuruoka and S. Ushioda, 

J. Elec. Micr. 53 (2004) 169. 

Four Fiber-bundles (NIMS) 

M. Sakurai, C. Thirstrup, and M. Aono, 

Applied Physics A 80 (2005) 1153. (~5%) 

Reasonable collection efficiency, 

but relatively difficult to align.

Fiber Tip Design―Near Field Collection Mode 



D. Fujita, et al. (NIMS), 

Micr. Res. Techn. 64(2004)403. 

T. Murashita (NTT) 

J. Vac. Sci. Technol. B 15(1997)32. 

Est: 100-fold better than lens system? 

Collection efficiency: ~10% 

The fiber tip is difficult to 

make. Expensive! 

Good coupling difficult!

UHV Photon STM: Double Lens Design 



Hemisphere collection efficiency: ~9% 

G. Hoffmann, J. Kroger, R. Berndt (Univ. Kiel) 

Rev. Sci. Inst. 73 (2002) 305. 

Reasonable 

collection efficiency 

(2−15%) 

+ 

Relatively easy 

To align 

Light collection systems 

used most often 

in the STML community!

UHV Photon STM: Photon Detection Systems 



Photon signals emitted from the STM tunnel junction is very weak! 

• Inelastic Tunneling (IET) Process: Small excitation efficiency 

• Nanoscale junction: Small excitation and emission area 

• Emission (angular) over hemi-sphere: Low collection efficiency 

Required conditions for photon detectors: 

• High sensitivity 

• Low dark counts (noise) 

Distinguish the signal above the noise! 

High S/N ratio! 

Ordinary photon detectors operating in the linear mode fail. 

X 

e.g., non-avalanche photodiode: the photon/electron ratio is one to one. 

Multiplier/Gain 

or 

Integration over Pixels 

Single Photon Counting? 

tip 

- 

+ e

Single Photon Counting Techniques 



Three Technologies: 

• Silicon Avalanche Photodiode (APD) 雪崩光电二极管 

Single Photon Counting Modules (SPCMs), 

1 photon → many electrons after avalanche effect, Gain = 1―1000 

high quantum efficiency (500-1100 nm, up to 70% at 650 nm), visible-infrared 

• Photomultiplier Tubes (PMT) & MCP-PMT 光电倍增管 

• Charge Coupled Devices (CCD) 电荷耦合器件 

Factors: 

• Wavelength 

• Sample size 

• Speed 

• Cost 

PerkinElmer-APD: Φ=200 μm, point-like nanoscale sample (focusing not good for large) 

fast response (count a photon every 50ns), low dark counts (Peltier cooled, 50 cps) 

photon → photocathode (photovoltaic effect) → many electrons, Gain = 105―106 115-900 nm, most sensitive for UV and blue (20%); low dark counts (3-50 cps) 

several mm – cm in size, wide range of sample size; 

very fast response (30ns), MCP-PMT (TTS=25ps + electronic shutter) 

An array of ordinary (non-avalanche) photodiodes, the amount of charge on capacitor 

300-1000 nm, up to 90% for LN2-cooled BCCD 

CCD pixel: 7μm, but array in mm; very wide range of sample size 

low dark count and noise, high SN ratio, but slow.



Data acquisition / Modes of Measurements 

Emission intensity 

STM-Induced Light Emission 

Bias and current dependency −Threshold, QE(η), excitation nature 

Photon maps −Threshold, quantum efficiency, spectral imaging map 

Isochromat spectra −Nature of particular emission feature, e.g, IET vs HET 

Angular distribution −Polarization effects, dipolar coupling 

Optical emission spectra 

λ-resolved luminescent spectra −Nature of opt. transition, interaction, ET… 

Bias dependence −Threshold, excitation behavior, excitation map 

Site dependency −Chemical mapping, spectral imaging map 

Time-resolved spectroscopy and photon statistics 

Lifetime −Nature of optical transitions and excited states; exciton decay kinetics 

Bunching vs antibunching −Photon emission statistics

Examples: 

Photon Map & Spectral Imaging Map 



G. Hoffmann, J. Kroger, R. Berndt 

Rev. Sci. Inst. 73 (2002) 305. 

topograph spectral imaging map 

Ag(111) at -3V, 5nA, 100ms/pixel 

10 5 cps at 5 nA →η= ~10 -4



Excitation Spectroscopy 

― Excitation Map 

G. Hoffmann, J. Kroger, R. Berndt 

Rev. Sci. Inst. 73 (2002) 305. 

Ag(111) 

V t=1.8−9.5 V, I t= 5 nA

Isochromat spectra 



―A support to the IET excitation mechanism 

Photon intensity ― Bias voltage 

at fixed current (It=5nA) & photon energy ( 2.1 eV, optical bandpass filter) 

Example: Cu(111) in the Proximity Field Emission Regime 

Resonance state 

Vn: 

Resonance-state energy 

Ep: 

Tip-induced-mode energy 

• Oscillation of dI/dV ↔ electron standing wave in the tip-sample gap (interference) 

• Isochromat spectra: Similar oscillatory behavior but with a constant offset of 2.2 eV! 

Field emission resonance states confined in the gap region 

Interaction of electrons with tip-induced modes occurs within the tunneling gap! 

Inelastic energy loss occurs within the gap → IET excitation mechanism! 

Berndt & Gimzewski 

Ann. Phys. 2(1993)133

Isochromat spectra 

―The “Universal” Intensity Maximum Issue 



Photon intensity ― Bias voltage 

at fixed current (It=5nA) & photon energy ( 2.1 eV, optical bandpass filter) 

Example: Cu(111) in the Proximity Field Emission Regime 

“Universal” intensity maximum at V ~ 3.7 V! 

In the tunneling regime: universal intensity maxima at Vb ~ 3 – 4 V, 

Independent of tip & sample material, wavelength of detected photons! 

Model: Competition of two counteracting mechanisms― 

Berndt & Gimzewski 

Ann. Phys. 2(1993)133 

Johansson, Monreal, Apell, PRB 42(1990)9210. 

1. Initial increase: growing number of energetically allowed channels; 

2. Drop above ~3.5 V: s-V relation! vertical tip retraction much stronger for 3.5−4V! 

→ Smaller field enhancement → Decrease in intensity!

3. Status of STM Induced Luminescence 



• Metals 

• Semiconductors 

• Molecules and Nanostructures 

• Theoretical models and calculations 

R. Berndt, Wittenberg 2004



Young et al. (1972) 

FE electrons 

↓ 

hν, sec. e− ↓ 

topography 

(NIST) 

R. Berndt, Wittenberg 2004 

of Junction Luminescence 

Lambe & McCarthy (1976) 

MOM junction, radiative decay of plasmons, IET 

Gimzewski et al. (1988) 

Isochromat Spectra: Ta, Ag 

↓ 

hν, local IPES 

(IBM Zurich)

Photon Emission from Metal Surfaces by STM 



R. Berndt, J. K. Gimzewski, P. Johansson, PRL 67(3796)1991 

Field emission regime Tunneling regime 

Photon emission 

dramatically enhanced! 

Strikingly different 

spectra: 

greatly red-shifted!



Extent: ~5 nm 

TIP: coupled, localized plasmons of both sample and tip! 

Symmetric/antisymmetric charge density oscillation 

Lowest-order mode―dipolar plasmon resonance 

Field enhancement G(ε) → Im(ε),damping 

η~10 -4 , IET vs HED 

Quantum cutoff: hν ≤eV


photon 

experiment theory 

Physical Review Letter 74(102) 1995 

First report atomic resolution 

in STM-induced photon emission! 

height 国家自然科学基金委员会



Uehara, Fujita, Ushioda, PRL 83(1999)2445; Uehara & Ushioda, PRB 66(2002)165420 

Recombination of d-band holes with sp electrons + LSP 

?



Phys. Rev. Lett. 93 (2004) 076102 

a: Au(110) topograph, b & c: photon map 

I: on top, II: below a step, III: in-between 

“No variation in spectral shape, 

except for a minute, rigid shift.” 

PRL 83(1999)2445



Photon Emission from 

Semiconductor Surfaces

Nanometer resolution in luminescence microscopy of III-V heterostructures 



D. L. Abraham, A. Veider, Ch. Schönenberger, H. P. Meier, D. J. Arent, and S. F. Alvarado 

First report: IBM Zurich: Appl. Phys. Lett. 56(1564)1990; JVSTB 9(1991)409 

Photon Map 

GaAs/Al x Ga 1-x As 

Quantum well/barrier heterstructures 

Renaud & Alvarado: Phys. Rev. B 44(1991)6340. 

Mechanism: p-type GaAs 

Conventional band edge luminescence: 

Electron-hole pair recombination 

between CV and VB (E g )!

Injection luminescence from CdS(1120) studied with scanning tunneling microscopy 



R. Berndt, et al., Physical Review B 45 (14095) 1992 

G: band-edge emission 

C: emission from deep levels 

Mechanism: 

-V: e-h pair recombination 

+V: impact ionization



A. Downes & M.E. Welland, Phys. Rev. Lett. 81 (1998) 1857 

Si(111)-7x7, 3.4 V, 30 nA 

Mechanism: Radiative decay of TIP modes! 

Lateral: 

Vertical: 

Plasmon wave function: 

Resolution of photon map: 

Lateral extent of plasmon modes 

~2 nm for R=3 nm and s=0.6 nm



C.Thirstrup, M. sakurai, K. Stokbro, M. Aono, Phy. Rev. Lett. 82 (1999) 1241 

Si(100)-3x1-H (-3 V, 8 nA) 

Spatially indirect dipole transition 

Between tip (DOS) and sample (DBs) ?



Photon Emission from 

Molecules and Nanostructures 

• 0D-Quantum dots 

• 1D-Quantum chains 

• 2D-Quantum wells 

• Organic molecules



Emission mechanism: 

the (1,0) mode of the Mie resonance in Ag nanoclusters 

Quantum size effect 

Size dependence of the Mie resonance (plasmons)! 

vs. shape transition?



Quantum confinement effect! 

MBE 

InAs QDs 

20-30 nm 

Photon Map



IET emission (TIP) Isochromat spectra 

hν=1.63 eV → = offset! 

LDOS of 1D-chain resonance states No direct interband transition from Ag chains!



No direct “interband” transition 

from 1D-Ag quantum chains on metal surfaces! 

How about 

2D-quantum wells on metal surfaces?

Luminescence from Metallic Quantum Wells in a Scanning Tunneling Microscope 



Germar Hoffmann,* Jörg Kliewer, and Richard Berndt 

Physical Review Letter 87(176803) 2001

Luminescence from Metallic Quantum Wells in a Scanning Tunneling Microscope 



Germar Hoffmann,* Jörg Kliewer, and Richard Berndt 

Physical Review Letter 87(176803) 2001 

3 

2 

1 

3 

2 

1 

q 

p 

p: plasmon 

modes 

q: quantum 

well state 

带间跃迁: 

OK!



STM Induced Luminescence from Molecules? 

hv 

tip 

- 

+ e- 

Excitation source: Highly localized tunneling electrons 

Single molecules DIRECTLY adsorbed on the metal surface 

Molecule based fluorescence from tunnel junctions? 

hv 

Feasibility?

Absorbance energy 

Light Absorption and Fluorescence 



Absorption=10 -15 s 

Fluorescence =10 -9 s 

Ground State 

Electrons 

S 2 excited state 

S 1 excited state 

Nonradiative 

dissipation 

Basics of molecular photophysics 

光致发光 

LUMO 

HOMO 

π* 

π

Organic Electroluminescence (EL) 



E F 

注入式电致发光 

OLED (Thin Film) 

+ 

h + 

LUMO 

HOMO 

LUMO 

HOMO 

exciton decay 

- 

e - 

ITO HTL EL ETL Mg 

Excitation of Electroluminescence: 

• Intrinsic EL: Field/impact excitation (>106 V/cm) 

• Injection EL: Excitation by the potential energy 

of opposite charge carriers (∼106 V/cm) 

Charge carrier: Injection, transport? Spin statistics? 

Excition: formation and decay? 

E F 

电致发光



Photon Map 

C60 on Au(110)

STM induced molecular luminescence 

• 1993, Spatially resolved photon map of molecules 

C60/Au(110) 

——R. Berndt, R. Gaisch, J. K. Gimzewski, et al., 

••• 

••• 

Science 262 (1993) 1425. 

No optical emission spectra! 

Nature of emission remains unclear. 

• 2002,Spectrally resolved photon maps; 

HBDC/Cu(111); 

Spacer! Submolecular resolution! 

—— Hoffmann, Libioulle and Berndt, 

Phys. Rev. B 65 (2002) 212107. 

Role of molecules? 

Spacer or direct emitter? 



IBM’s need 

for sharper display!



Photon Emission from HBDC on Cu(111) with submolecular resolution 

G. Hoffman, L. Libioulle, R. Berndt, Phys. Rev. B65 (2002) 212107 

• Spectra: Blue-shifted 

• Fluorescence: quenched! 

• Role of molecules: “Spacer”!

Photon emission from monolayered porphyrins on Cu(100) 



H2TBPP Porphyrin 

H 

H 

H 

H 

H H 

H H 

NH 

N 

H 

H 

N 

HN 

H 

H 

H 

H 

H 

H 

Z.-C. Dong, et al., Surf. Sci. 532 (2003) 237 

STL Intensity (cps/nA) 

1.5 

1.0 

0.5 

0.0 

ML-H2TBPP/Au(100), -2.9 V, 0.1 nA 

Au(100), -3.0 V, 10 nA 

Theory, 3 V, R = 50 nm, d = 0.4 nm 

Blue-shift 

400 450 500 550 600 650 700 750 800 

Wavelength Sample Bias (nm) (V) 

ω 1 ∝ω p (d/8R) 1/4 

Monolayer case: Role of molecules: Spacer!



Photon Emission from Organic Molecules? 

Plasmon mediated emission 

+ 

Molecular fluorescence 

Coupled / mixed! Fluorescence quenching! 

Complication! 

Role of molecules? 

Spacer or direct emitter? 

Evidence to look for: Distinct molecular-specific signals!

分子荧光复杂性根源: Nonradiative Energy Transfer near Metal Surfaces 



Fluorescence Quenching 

or, dipole quenching 

Nonradiative damping 

ε 0 

Molecule 

ε 

Metal 1 

Substrate 

Dipole 

d 

Dipole 

Image 

dipole 

Classical electromagnetic theory: 

For large distances, the surface acting as a mirror 

to cause electric field interference, 

the luminescent lifetime oscillates as a function of d. 

τ 

0 250 500 

d (nm) 

For small distances (

Strategy to generate molecular fluorescence 



Decoupling & nanoprobe excitation 

_ 

Nanoprobe 

Strong field enhancement at the tip apex 

Emissive layer 

Decoupling layer 

Substrate 

Motivation: to avoid fluorescence quenching…

Vibrationally resolved fluorescence excited with submolecular precision 



X. H. Qiu, G. V. Nazin, W. Ho: Science, 299 (2003) 542 

ZnEtioI/Al 2O 3/NiAl(110) 

Use of an oxide spacer!

Vibrationally resolved fluorescence excited with submolecular precision 




LUMO+1 → LUMO! 

Anionic fluorescence!




LUMO+1 → LUMO! 

Anionic fluorescence! 

? 

Can we generate by STM 

conventional fluorescence 

from neutral molecules 

due to 

LUMO → HOMO transition?



Distinct fluorescent peak: Molecular specific! 

Clear molecular origin!



• Bipolar operation ! → 

Electrons injected into LUMO at +/- bias! 

• Double-barrier junction model! 

• Two processes: 

LSP + Molecular Fluorescence 

• Anomalous upconversion EL: 

Hot- electron excitation + 

Franck-Condon radiative decay 

hν > eV !!! Why? Z.-C. Dong, et al. PRL 92 (2004) 086801.

Decoupling and Charge Carrier Confinement: pn-type Heterostructuring 



Photoluminescence (a.u.) 

5000 

4000 

3000 

2000 

1000 

PL, ex 442 nm, He-Cd laser 

STML, +25 V, 2 nA 

658 

0 

0 

400 450 500 550 600 650 700 750 800 

Wavelength (nm) 

723 

H 2TBPP 

TPD 

Cu(100) 

250 

200 

150 

100 

50 

STM Luminescence (a.u.) 

- 

+ 

+ 

e − 

Substrate surface 

Tip 

Dong, et al.: PRL 92, 086801(2004); APL 84, 969(2004); PRB 70, 233204(2004); SRL 13(2006)143. 

- 

+



C 60 /NaCl/Au(111)



STM Photon Emission 

Model and Theory?

E F 

E F 

hv 

hv 

E F 

surface 

states 

E F 

Models? 



Tip-induced-plasmon (TIP) Electron-hole recombination 

Metals Semiconductors (direct Eg ) 

Tip Sample 

E F 

E F 

hv 

EC EF EV hv 

LUMO 

HOMO 

Tip Sample 

Spatially indirect dipole transition Intramolecular radiative transition 

Semiconductors (indirect Eg ) 

Molecules 

E F

Questions? 



Which part, electrons or photons, influences 

the light emission spectrum most? 

Depends on the system! 

• Metals: photons (plasmons) 

• Semiconductors: electrons (bandgap, etc.) 

• Quantum wells: both 

• Molecules: < 1ML, photons; decoupled: both?



Theoretical Models 

Radiation Field Electrons 

H = H el-n + H L + H s 

H s ↔ A•p 

P. Johansson

2 3 

o 

G ( θ, r ′ , ω) 

Local field enhancement 

(Boundary charge method) 

Radiated power 



2 2 

d P ω 

2 

3 

= ∑ d r′ G ( θ, ′ , ω) ⋅ jif 

( ′ , ω ) ⋅δ 

( Ei− E f − ω) 

dΩd( ω) 4π 

ε c ∫ r r 

h 

h 

i, 

f 

z 

G z jif 

P. Johansson et al., PRB 42, 9210 (1990) 

d 

ε tip 

b 

ε sample 

j 

φ 

θ 

hω 

hω 

j ( r ′ , ω ) 

if 

Current density 

(Tersoff & Hamann extended) 

e 

V b 

hω 

Tip induced plasmon (TIP) modes for a metal tip on a metal surface 


J. Aizpurua et al., PRL 89, 156803 (2002)



G = G(ε, R, d), sensitive to the tip status! 

Molecules: No theory for STML yet! 


J. Aizpurua et al., PRL 89, 156803 (2002)



Summary 

• STM induced luminescence (STML): 

-Photon emission out of inelastic tunneling processes! 

(高度局域化隧穿电子 → 非弹性隧穿 → 光子发射!) 

-Various information on the nature of tunnel junctions and transitions 

-Good tool for studying optoelectronic behavior of single molecules 

• Factors affecting STML: 

-Geometrical: R, d, ε! (几何因素) 

Elastic tunneling current is also important: I t ↔ d ↔ TIP ↔ IET! 

-Electronic: Initial and final DOS! (电子结构因素) 

Energy level matching (DOS of resonance states/hybrid states) 

• Status of the STML research: 

-Metals and semiconductors: relatively mature; 

-Molecules: many unsolved issues! active and exciting! 

e – 

in the space, energy, and time domains (单分子光电行为) 

(molecular scale electronics and optoelectronics) 

+ 

_ 

hv

Perspectives: 

• Role of molecules? 

• Role of substrates (plasmons)? 

• Coupling among electron, exciton, plasmon, phonon, photon? 

• Raising quantum efficiency (OLED)? Light sources for imaging? 

• Single molecule electroluminescence? Single photon sources? 

• Lifetime measurements? Luminescence decay kinetics? 

• Fluorescence IETS? Chemical mapping? 

• Spin injection STML? Spin decay kinetics (population)? 

Near-field detection 



Coupled 

system! 

Far-field detection 

• Quantum tuning of photonic states for molecular optoelectronics? 

Molecule 

Tip 

Fiber 

- 

+ 

Substrate 

Dipolar 

radiation



Thank you !

扫描隧道显微镜诱导的发光 - 中国科学院物理研究所

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