Photodissociation and Photoionization Mechanisms in Photodissociation and Photoionization Mechanisms in Lanthanide-based Fluorinated Lanthanide-based Fluorinated ββ-diketonate MOCVD -diketonate MOCVD PrecursorsPrecursorsJiangchao CHEN, Robert J. WITTE, Yajuan GONG, Qingguo MENG, P. Stanley MAY, Mary T. BERRY*Department of Chemistry, University of South Dakota, Vermillion, SD 57069

Experiments Experiments

Understanding the photochemistry of gas-phase

metal-organic compounds is fundamental to

harnessing the full potential of laser-assisted metal-

organic chemical vapor deposition (LCVD). A

detailed photodissociation mechanism for the

lanthanide-based MOCVD precursors LnL3

[L=1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-

octanedionate (fod-), hexafluoroacetylacetonate

(hfac-)] and Ln(hfac)3diglyme was developed using

Nd:YAG or OPO (optical parametric oscillator) laser

induced photoionization time-of-flight (TOF) mass

spectrometry. The collisionless environment of the

molecular beam source revealed a series of

unimolecular steps starting with dissociation of an

intact -diketonate ligand. Dissociation steps for the

second and third ligands are each potentially

associated with deposition of a fluoride on the metal,

leading to one of three ultimate products Ln, LnF, or

LnF2.

Introduction Introduction

References References

Pollard, K. D.; Jenkins, H. A.; Puddephatt, R. J.Chem. Mater. 2000, 12, 701. Ow, F.P.; Berry, M.T.; May, P.S.; Zink, J.I. J. Phys. Chem. A 2006, 110, 7751

Meng, Q.G.; Witte, R.J.; May, P.S.; Berry, M.T. Chem. Mater. 2009, 21, 5801

Results of LnL3 Results of LnL3 Results of Hhfac Results of Hhfac Results of EuL3Diglyme@412nm Results of EuL3Diglyme@412nm

Conclusion ConclusionIn laser-assisted chemical vapor deposition using metal-organic precursors, the nature of deposited materials in strongly influenced by unimolecular gas-phase reactions.

The fluorination process proposed during the photodissociation is in competition with production of bare metal Ln(0) which is thought to proceed through a mechanism of three-fold intact-ligand dissociation mediated by photo-excitation to dissociative regions of LMCT states.

Acknowledgements AcknowledgementsDr. M.T. Berry and Dr. P.S. MayChemistry Department, Univserisyt of South DakotaNSF-EPSCoR

Results of LnL3Diglyme Results of LnL3Diglyme

HfodHhfac

Pulsed Valve Sample

Holder

Acceleration Grid

Skimmer

Flight Tube

Repeller Plate

Extraction Grid

Laser

Cooling Collar

Diffusion Pump Viewing

Port

MCP Detector

Turbo Pump

Flight Tube

Carrier Gas

Scheme 1. Experimental diagram for photo-ionization time-of-flight mass spectrometer, PI-TOF-MS.

Scheme 1. Experimental diagram for photo-ionization time-of-flight mass spectrometer (PI-TOF-MS)

Diglyme

Diglyme

Figure 1. PI-TOF-Mass spectra for Eu(hfac)3 (top) and Eu(fod)3 (bottom) at 532 nm (200 mJ/pulse). The dominant ions are Eu2+, Eu+, and EuF+ in both figures, together with the much weaker feature for EuF2

+.

Figure 2. PI-TOF-Mass spectra of Pr(hfac)3, Eu(hfac)3 and Gd(hfac)3 at 355 nm.

Scheme 2. Sequential dissociation of intact neutral ligands following photo-excitation to repulsive regions of a ligand-to-metal charge-transfer state.

CHCC

F3C

O

CF3

O

Eu C-C bond rotation-CO, -EuF

F2C CH C CF3

O

CHCC

O

F3C

CF3

O

Eu

160 m/z fragment weak peak

F2C CH C O + CF3

CF369 m/z fragment

strong peak

FC C C O-HF

C C C O 52 m/z fragment strong peak

FC C C O71 m/z fragment

strong peak

-F

FC C43 m/z fragment

strong peak

Scheme 3. A proposed fragmentation mechanism along with annotation regarding observation of the individual fragments in the PI-TOF-mass spectrum.

Figure 3 PI-TOF-Mass spectra of Eu(hfac)3 and H-hfac@532 nm. The peaks at 43, 52, 71 amu are characteristic fragments from Eu(hfac)3, and are weak or absent in the spectra of Hhfac and Ln(thd)3. The observation of every charged species in the proposed photofragmentation mechanism, provides a convincing argument in support of the mechanism in Scheme (3), though the fluorination may not occur until after the first hfac ligand has dissociated.

Figure 4. PI-TOF-Mass spectra of Pr(hfac)3diglyme at 532 nm and 266 nm.

Figure 5. PI-TOF-Mass spectra of Gd(hfac)3diglyme and Eu(hfac)3diglyme at 532 nm and 266 nm respectively.

Figure 6 PI-TOF-Mass spectra of Eu(hfac)3diglyme @412 nm. The Eu species shows a parallel path in which the second hfac ligand dissociates without depositing fluoride. The right panel shows an expanded scale, revealing the parallel paths to EuF and EuF2.

Eu(hfac)diglyme EuFdiglyme EuF

Eu(hfac)3diglyme Eu(hfac)2diglyme

EuF(hfac)diglyme EuF2diglyme EuF2