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
[email protected] 17, 2008
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)
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)