Supporting Information

Niobium Pentoxide: A Promising Surface-Enhanced Raman

Scattering Active Semiconductor Substrate

Yufeng Shan,†‡ Zhihui Zheng†‡, Jianjun Liu †‡ Yong Yang,†‡ Zhiyuan Li,§ Zhengren

Huang,†‡ and Dongliang Jiang†

†State Key Laboratory of High Performance Ceramics and Superfine Microstructures,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road,

Shanghai 200050, China.

‡Graduate School of the Chinese Academy of Sciences, Beijing, China

§Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China

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Figure S1. The SEM images of (a) Nb2O5, (b) TiO2 and (c) Ag substrates.

Figure S2. The UV-vis adsorption spectra of analytes solutions before and after

adsorption by Nb2O5 powders for (a) MeB, (b) MV and (c) MB.

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Figure S3. The Raman spectra of MeB collected from six arbitrary Nb2O5

semiconductor substrates. For each substrate three different spots are randomly

selected.

Figure S4. (a) SERS signals of MV molecules with various concentrations excited by

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the laser of 532 nm on Nb2O5 substrates. (b-c) Raman spectra of solid MV powders

and MV molecules with the concentration of 510-5 M collected on Nb2O5 substrates

under the excitation of 532 and 633 nm lasers, respectively.

Figure S5. (a) SERS signals of MB molecules with various concentrations excited by

the laser of 532 nm on Nb2O5 substrates. (b) Raman spectra of solid MB powders and

MB molecules with the concentration of 510-5 M collected on Nb2O5 substrates

under the excitation of 532 nm laser.

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Figure S6. Energy level scheme of (a) MeB, (b) MV, (c) MB on Nb2O5 and the

corresponding photo-induced charge transfer (PICT) process between adsorbed

molecule and Nb2O5 substrate under the irradiation of 532 nm laser.

Figure S7 (a) The UV-vis optical absorption spectrum of pure Nb2O5 powders, MV

modified Nb2O5 powders and MV aqueous solution. (b) The UV-vis optical absorption

spectrum of pure Nb2O5 powders, MB modified Nb2O5 powders and MB aqueous

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solution.

Scheme S1. Structure of analyte molecules of (a) MeB, (b) MV and (c) MB.

Scheme S2. (a) Three kinds of interaction sites with dye molecules on Nb2O5 surface.

(b) The negative net charges on the surface for Nb2O5 in alkalescent dye solutions.

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Table S1. Raman frequencies and corresponding assignments of the main bands

observed from the MeB solid powders and the SERS spectra by the excitation at 532

nm, 633 nm and 780 nm.[1]

Assignment Solid MeB

= 532 nm

Frequency,

cm-1

MeB-Nb2O5

= 532 nm

Frequency,

cm-1

Solid MeB

= 633 nm

Frequency,

cm-1

MeB-Nb2O5

= 633 nm

Frequency,

cm-1

Solid MeB

= 780 nm

Frequency,

cm-1

MeB-Nb2O5

= 780 nm

Frequency,

cm-1

(C-C)ring 1630 1632 1624 1622 1614 1621

(C-N)asym 1445 1441 1424

(C-N)sym 1370 1402 1398 1391 1394 1413

1307 1305 1298

(C-H) 1161 1155 1160 1151

1080 1077

(C-H) 1042 1042 1030 1035

955 957 946

866 864 890 894

832

813

775 774 773 770 767

(C-H) 674 667 667

(C-S-C) 618

(C-S-C) 597 601 598 592 598

564 554

(C-N-C) 506 504 505 496

466 472

(C-N-C) 453 453 449 442 447

(Nb-N) 252 250 247

Table S2. The calculated RSD values for the main Raman peaks of MeB molecules

collected on Nb2O5 substrates.

Peak Position

(cm-1)

453 504 601 674 714 1042 1077 1397 1441 1631

RSD 0.04 0.03 0.05 0.09 0.05 0.07 0.06 0.04 0.04 0.06

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Calculation of the enhancement factors （ EFs） . The EFs were calculated

according to the formula:

(1)

(2)

(3)

In formula (1), ISERS and IRaman are the integrated intensities of the selected peak in the

non-SERS spectra of solid analyte powders and SERS spectra collected on the Nb2O5

substrates, and NSERS and NRaman are the corresponding average number of molecules in

scattering area for non-SERS and SERS spectra. In formula (2), , Sst, M and h refer

to the density of solid analytes, scattering area of irradiation laser, molecular weight

of analytes, and the confocal depth of the laser beam, respectively, while NA is the

Avogadro constant. In formula (3), Nad is the number of adsorbed analyte molecules to

Nb2O5 powder estimated from the adsorption curves in Figure S2, and S t is the total

surface area of all the Nb2O5 NPs, where the average size of 104 nm is adopted.

Accordingly, ;

is the adsorption ratio evaluated from the adsorption curves in Figure S2.

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So formula (1) can be rewritten as:

For MeB analyte, is 1.757 g/cm3, is 19.8%, M is 319.86 g/mol, and

For MV analyte, is 1.109 g/cm3, is 13.9%, M is 393.96 g/mol, and

For MB analyte, is reasonably evaluated to be 2 g/cm3, is 1.8%, M is 799.8

g/mol, and

h increases with increase of the incident laser wavelength, and for 532, 633 and 780

nm lasers, h is estimated to be ~25 μm,[2] ~100 μm,[3] and ~200 μm.[4] By utilizing

these parameters, the EFs with various excitation lasers for each analyte molecules

can be obtained.

The calculation of the contribution of photo-induced charge transfer (PICT) to

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the molecular polarizability tensor in the semiconductor-molecule system

According to the Herzberg-Teller theory, the vibrational coupling of the conduction

band states |S and the valence band states |S' with the molecular ground states |I and

excited states |K is considered in the semiconductor-molecule system[5]. In our

system, the condition of ωSS'>ωIK is considered. The intensity of a Raman transition

(IRaman) could be expressed as the function of the molecular polarizability tensor:

Where I0 is the intensity of incident laser with an angular frequency of ω0, and ωIF is

the molecular transition frequency between states I and F (presumably two different

vibrational levels of the ground electronic state).

Considering the second-order perturbation, the polarizability tensor () could be

written as:

Where K represents all the other states of the molecule, μ is the dipole moment

operator, the μ and μ are the three-dimensional component in space (X, Y, Z). Taking

the zero-order Born-Oppenheimer approximation into account, all the states (I, K, F)

as the products of the electronic and vibrational wave functions can be written as:

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Where the subscript e indicates a pure electronic state, and lowercase letters represent

the vibrational functions. Based on the Herzberg-Teller theory that even small

vibration may cause mixing of zero-order Born-Oppenheimer states, so for the

semiconductor-molecule system:

Where zero refers to the zero-order Born-Oppenheimer states, HeN is the electron-

nuclear attraction term in the Hamiltonian, evaluated at the equilibrium nuclear

positions (0). |Se and |S'e indicates the electronic states lying in the conduction band

and valence band of semiconductor, respectively. hKS is the coupling matrix elements

representing the degree to which a particular vibration Q can mix state Se with the

state Ke. Similarly, hIS' is the coupling matrix element representing the degree to which

a particular vibration Q can mix states S'e with state Ie. For the purely electronic

transition moment between states, we write:

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Next, we consider the situation that the energy gap between |S and |S' state for

semiconductor is larger than that between the |K and |I state for molecules. The

polarizability tensor in the semiconductor-molecule system can be rewritten as

follows (assuming that ω0>ωSS'):

= A+B+C

A represents the contribution of the molecular resonance to the polarizability tensor

via MIK.

B represents the contribution of photo-induced charge transfer of molecule-to-

semiconductor to the polarizability tensor via MIS. The transition borrows intensity

from the allowed transition MIK by means of vibrational coupling between the excited

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molecular state K and semiconductor conduction band states S though the matrix

element hKS.

C represents the contribution of photo-induced charge transfer of semiconductor-to-

molecule to the polarizability tensor via MS'K. The transition borrows intensity from

the allowed transition MIK by means of vibrational coupling between the molecular

ground state I and semiconductor valence band states S' through the matrix element

hIS. This is illustrated in Scheme. S3(a).

Next, the situation that the frequency of exiting light is far from the semiconductor

electronic transition frequency (ωIK<ω0<ωSS') is considered. In term of A, the state K is

actually a semiconductor conduction band state S or the state I is actually a

semiconductor valance band state S'. Under this consideration the similar molecular

resonance contribution to the polarizability tensor can be expected. In this case, the

sum cannot be carried out over vibrational states k, and a single vibration will

dominate the sum. The polarizability tensor can be written as follows:

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Ar is ascribed to the molecular resonance to the polarizability tensor via M IK. Am

represents similar molecular resonance contribution due to the photo-induced charge

transfer of molecule-to-semiconductor, which is from the molecular ground state I to

one of semiconductor conductor band states S. B' represents the contribution of the

photo-induced charge transfer of molecule-to-semiconductor, which is from the

molecular ground state I to one of the semiconductor conduction band states S. This

transition obtains its intensity via MIS through intensity borrowing from the (assumed)

allowed transition IK. The borrowing mechanism is vibrational coupling through hKS

which represents coupling of the semiconductor conduction band states S to the

molecular excited state K through some vibrational mode Q. This is illustrated in

Scheme S3(b). As represents similar molecular resonance contribution due to the

photo-induced charge transfer of semiconductor-to-molecule, which is from one of

semiconductor valence band states S' to the molecular excited state K. C' represents

the contributions of the photo-induced charge transfer of semiconductor-to-molecule,

which is from one of the semiconductor valence band states S' to the molecular

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excited state K. This transition obtains its intensity via MS'K through intensity

borrowing from the (assumed) allowed transition IK. The borrowing mechanism is

vibrational coupling through hIS' which represents coupling of the semiconductor

valence band states S' to the molecular ground state I through some vibrational mode

Q. This is illustrated in Scheme S3(c).

Scheme S3. (a) The scheme for the photo-induced charge transfer of semiconductor-

to-molecule and molecule-to-semiconductor under the situation of ω0>ωss'. (b) The

scheme for the photo-induced charge transfer of semiconductor-to-molecule in

semiconductor-molecular system under the situation of ωIK<ω0<ωss'. (c) The scheme

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for the photo-induced charge transfer of molecule-to-semiconductor in

semiconductor-molecular system under the situation of

ωIK<ω0<ωss'.

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shaped silver nanoparticles. Chem. Phys. Lett. 447(4–6): 305-309 (2007).

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Raman scattering from platinum electrodes using a confocal Raman microscope: dependence

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[3] N. J. Everall. Confocal Raman microscopy: common errors and artefacts. Analyst 135(10):

2512-2522 (2010).

[4] A. Macdonald, A. Vaughan. Numerical simulations of confocal Raman spectroscopic depth

profiles of materials: a photon scattering approach. J. Raman Spectro. 38(5): 584-592 (2007).

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