Structure Determination of Silicon Clusters in the Gas Phase

A Vibrational Spectroscopy and DFT Investigation

Jonathan T. Lyon, Philipp Gruene, Gerard Meijer, André Fielicke, Ewald Janssens, Pieterjan Claes, and Peter

Lievens

lyon@fhi-berlin.mpg.deJune 17, 2008

Why Study Silicon

Clusters?

The Importance of Si

►Silicon is important to the electronics industry

►Current trend toward miniaturization

►Predictions that silicon clusters do not have bulk-like structures

*Ho, K.-M.; Shvartsburg, A. A.; Pan, B.; Lu, Z.-Y.; Wang, C.-Z.; Wacker, J. G.; Fye, J. L.; Jarrold, M. F. Nature 1998, 392, 582.

Silicon Nanowires

►Silicon nanotubes potentially an assembly of clusters

*Marsen, B.; Sattler, K. Phys. Rev. B 1999, 60, 11593.

What is Known About Sin?

►Vibrational spectroscopy in matrices1

►Ion-mobility experiments2

►Photoelectron spectroscopy3

►UV Photodissociation spectroscopy4

1Honea, E. C.; et al. J. Chem. Phys. 1999, 110, 12161. 2Shvartsburg, A. A.; et al. J. Chem. Phys. 2000, 112, 4517.

3Cheshnovsky, O.; et al. Chem. Phys. Lett. 1987, 138, 119.

4Antonietti, J. M.; et al. Phys. Rev. B 2003, 68, 035420.

IR Action Spectroscopy

IR multiple photon dissociation(IR-MPD)

IR resonance enhanced multiplephoton ionization (IR-REMPI)

(i) Processes following IR multiple photon excitation

(ii) Detecting photon absorptions via the messenger technique

- Weak interacting messenger

- BDE typically <1000 cm-1

- Can be coupled with mass selection

IR-MPD Spectroscopy

►Vibrationally excite a molecule until part of it fragments (or fragment a weekly bound ligand)

►Need an intense radiation source in the region or molecular vibrations

hd

hh

E

IVR

►resonant absorption

►fast intramolecular vibrational redistribution (IVR), tIVR << 1 ns

►absorption of the next photon

►etc.

Free Electron Lasers

►Wavelength (λ) depends on:►kinetic energy of the electrons (E)

►undulator period (λu)

►magnetic field (K)2

22

1

)1(2

cm

E

K

e

U

Free Electron Laser for Infrared eXperiments (FELIX)

► Tunable between 40 and 2000 cm-1

► ≤ 100 mJ per macropulse

(1010 W/cm2 in a micropulse)

Experimental Setup

resonant absorption

fragmentation of the complex

Silicon Cluster Cations

►Si cluster cations studied by tagging with Xe (Ar will not bind to the clusters)

►Good IR spectra of Sin+ clusters in the n=6-20 range

200 300 400 500 600200 300 400 500 600 200 300 400 500 600

Wavenumbers (cm -1)

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

The Structure of Si8+

►Theory predicts new structure (edge-capped pentagonal bipyramid) for Si8+

►Lowest in energy

►IR spectrum fits the best

69

12

17

kJ/molhigher inenergy

Structures Identified

►Change from pentagonal bipyramid to trigonal prism building block occurs between Si9+ and Si10

+

Si6+

Si11+ Si13+ Si14+

Si7+ Si8+ Si9+ Si10+

Si15+

A Complete Story?

►Not all sizes have been identified, e.g., Si12

+

►Have fun theoreticians

200 300 400 500

0

0

10

200

10

200

0

0

100

100

10

0

10

010200

10

20

0 kJ/mol

+11

+22

+22

+23

+25

+26

+28

+33

+46

+63

Si12+-Xe

Inte

nsi

ty

Wavenumbers (cm-1)600

12-a

12-b

12-c

12-d

12-e

5

5

5

12-f

12-g

12-h

12-i

12-j

12-k

12-a

12-b

12-c

12-d

12-e

12-f

12-g

12-h

12-i

12-j

Conclusions

►IR-MPD is a suitable technique to obtain the vibration spectra of Sin+ in the n=6-23 range►Determination of cluster geometry is possible by

comparing experimental spectra with theoretical predictions

►Structures identified for Si6-Si11, Si13-Si15, and Si18

►Si8+ has a novel structure previously not considered

►Transition from pentagonal bipyramid to trigonal prism building blocks occur between Si9+ and Si10

+

Acknowledgments

►Research Group►André Fielicke►Philipp Gruene►Gerard Meijer

►Collaborators►Peter Lievens – K. U. Leuven►Ewald Janssens►Pieterjan Claes

►FELIX Staff►Britta Redlich►Lex van der Meer

►Funding and Equipment►FELIX ►Alexander von Humboldt Foundation

Other FELIX Users

►Robert C. Dunbar MG01►Monday June 16th @ 1:30 PM

►“AMINO ACID COMPLEXES OF METAL IONS WITH +2 CHARGE. DETERMINATION OF SUCCESSIVE LIGAND CONFORMATIONS BY IRMPD SPECTROSCOPY”

►Anouk M. Rijs RD08►Thursday June 19th @ 10:54 AM

►“ROTAXANES: MOLECULAR MOTION IN THE GAS PHASE”

►J. M. Bakker RG05►Thursday June 19th @ 2:51 PM

►“FIRST EXPERIMENTS AT FELICE, THE FREE-ELECTRON LASER FOR INTRA-CAVITY EXPERIMENTS”

Gas Phase Spectroscopy

More sensitive and selective

►Mass spectrometric detection of absorption

►Changes of the charge state = ionization

►Changes of particle mass = dissociation via absorption of multiple IR photons (IR-MPD)

►Problem: particle density in gas phase

►Direct measurement of absorption►Not sensitive enough (low particle density)►Not species specific (broad cluster distribution)

IR-MPD of Vanadium Clusterstime-of-flightmass spectrometer

no FELIX 7,08,0

7,1

7,2* * *

VxAry+

gas pulse(Xe/He)

metal rod/ silicon plate

Nd:YAG laser

liq. N2

resonant absorption

Fragmentation of the complex

depletion spectrumabsorption spectrum

FELIX at 32m(313 cm -1)

FELIX beam

Free Electron Laser for Infrared eXperiments (FELIX)

► Tunable between 40-2000 cm-1

(≤ 4000 cm-1 on 3rd harmonic)► ≤ 100 mJ per macropulse

(1010 W/cm2 in a micropulse)

Theoretical Chemistry

►Density Functional Theories

(B3LYP, BPW91, BP86)►Predict stable isomers►Compute infrared spectrum►Compare to experiment for structural assignment

experimental spectrum

calculated IR spectra for

different structures

structure assignment