Methods to produce andstudy clusters

http://fielicke.lmsu.tu-berlin.de/

André FielickeInstitut für Optik und Atomare Physik

Technische Universität Berlin, Germany

Program

1. What are clusters and why to study them?2. Making and characterizing free clusters3. Probing the structures

1. What are clusters and why to study them?

Clusters

Oxford English Dictionary:

1. A collection of things of the same kind, as fruits or flowers, growing closely together; a bunch.

a. Originally of grapes (in which sense bunch is now the usual term).

b. Of other fruits, or of flowers; also of other natural growths, as the eggs of reptiles, the air-cells of the lungs, etc.

Cluster compounds• Thermodynamically and kinetically stable

• Chemical synthesis in bulk quantities

• Characterization with “classical” spectroscopic methods (IR, NMR, XRD etc.)

Isolated clusters• Generation through aggregation of the

(atomic or molecular) constituents

• (Nearly) free choice of size (n), composition (n/m) and charge (z): MnLm

z

• Most clusters are not stable towards aggregation

• Experimental investigations are usually performed in the gas phase Molecular beam techniques

Co4(CO)12

H6O13+

(“Zundel” cation)

B12H122-

V8+ B16

PRL 93 (2004) 023401 JCP 137 (2012) 014317

Clusters of atoms and molecules

• multiples of a simple subunit, e.g. Cn, Arn, or (H2O)n

• The cluster size n can vary and determines the properties

• small clusters have (nearly) all atoms on the surface

Number of atoms

Surface atoms

radius [nm] 1 10 102

1 10 10310 104 105 106 107 1082

10 102 103 105104

Clusters Nano-crystals

5 6 7 8 9 10 11 12 13

Nbn+

?

Volume and surface of a cluster with n atomsSpherical cluster approximation

R

2r

3

34 RV

24 RS

Assignment1.1 How many Krypton atoms are in a spherical cluster of a) 1 nm, b) 10

nm, c) 100 nm radius? Assume that a single Kr atom fills an effective volume with 0.2 nm diameter in this cluster.

1.2 What is the ratio for surface vs. volume atoms for these clusters? The surface atoms contribute to the cluster surface by only ¼ of their ‚atomic surface‘, ¾ point toward the inside of the cluster.

Clusters: nano and smaller

Effects at the (sub)nano-scale quantum confinementlarge surface/volume ratio

structural changes

Emergence of new properties e.g.: magneticoptical / luminescence

chemical / catalytic

Size dependence of properties: Each atom counts

Magnetism

Stability

Fen + H2

Reactivity

Ionization energy

Motivations for the study of free metal clusters

Fundamental aspectsHow are properties emerging when going from the atom to the bulk?

Reference systemsTest and further development of theoretical methods

New materialsInspiration from particularly stable clusters

Model systems in heterogeneous catalysis

model

application

2. Making and characterizing free clusters

Bonding in clusters

dispersion

induction

dipole/dipole

ion/dipole

metallic

covalent

ionic

increasingbond strength

and cluster stability

1s 1s

He HeHe2

EB≈100 neV

EB≈4-6 eV

Cn

C=C=C or C≡C–C

EB≈7-8 eV(Na+Cl-)n

EB≈1-5 eV + +

+

++

+

+-

---

-

-

-

Experimental techniques for Cluster studies

Cluster production: Top-down vs. Bottom-upSputtering or Aggregation of the constituents

cooling

condensation

clustersbulk material

supersaturated vapor

vaporization

sputtering

Cluster productionSupersonic expansion of a gas

Adiabatic and isenthalpic expansion leads to strong cooling formation of a cold supersonic beam

Cluster formation via 3-body collisions near the nozzlee.g. Ar + Ar + Ar Ar* + Ar2 (conservation of energy and momentum)Dimers are condensation nuclei for larger clusters

Seeded molecular beam: cooling of the internal degrees of freedom

Cluster ProductionGas aggregation

(thermal) evaporation into a cold gas

Smoke source for the production of C60, C70 and larger carbon clusters

Typical vapour pressures of ~10-2

mbar need to be reached

(°C)Na 289Al 1217Ag 1027Au 1397

Cluster ProductionLaser ablation

heating of a small surface part of a solid target by a focused, intense short-pulse laser (typically Nd-YAG, 532 nm)formation of a plasma that contains ions and electronscooling with rare gas induces aggregation formation of neutral and charged (anionic and cationic) clusters

Converts practically any solid into clusters, very frequently used!Can be easily combined with reaction or thermalization channels, etc.

see: M.A. Duncan, Rev. Sci. Instr. 83 (2012) 041101.

A molecular beam cluster experiment

Mean free path length (identical particles)

Experiments under collision-free conditions

Vacuum range Pressure in mbar Molecules / cm3 mean free pathAmbient pressure 1013 2.7*1019 68 nmMedium vacuum 1-10-3 1016-1013 0.1-100 mmHigh vacuum 10-3-10-7 1013-109 10 cm - 1 kmUltra high vacuum 10-7-10-12 109-104 1 km-105 km

Sourcechamber

big pump

smallpump

time-of-flightmass spectrometer

VUV laser

operators

fore vacuumpump

infr

ared

lase

r bea

m fr

om F

EL

Metal cluster lab at the FHI-FEL in Berlin

Mass spectrometric characterizationIonization techniques for neutral clusters

Electron impactEfficient ionization at 60-100 eVIonization potentials (IPs) are onthe order of 5-15 eVexcess energy leads to fragmentationand changes mass distribution

Photoionization

UV lasers (nm) E (eV)Nd-YAG, 3rd 355 3.5Nd-YAG, 4th 266 4.7

KrF 249 5.0ArF 193 6.4F2 157 7.9

Nd-YAG, 9th 118 10.5single photon resonant multi photonspecies and state selective

kinB EEh

Mass spectrometric characterizationTime-of-flight mass spectrometry

acceleration of charged particles (ions) in an electric fieldparticles having the same charge but different mass are accelerated to the same kinetic energy

Measurement of the arrival time on the detector gives mass informationtypical experimental conditions: s=1 cm, D=10-300 cm,

E=100-10000 kV/cmA single mass spectrum can be measured within 5-100 µs.Mass resolution up to 10 000 amu can be achieved

2

2mvzeEs

mzeEsv 2

sDDzeEsmt

2

Example: Cobalt cluster cations produced by Laser ablation

Mass spectrometric characterization

time-of-flight (µs) time-of-flight (µs)

inte

nsity

(arb

. uni

ts)

mass resolution: 3402 2/1

max ttR

2. Dirty Terbium clusters, what is in there?

Other types of mass spectrometersI Magnetic sector field

II Quadrupolemoderate to high (104) resolution experiments on beams of mass selected ions (MS/MS)

III Ion traps, FT-ICR (ion cyclotron resonance) very high resolution (106), long storage timessimultaneous detection of all ionsexpensive

I-II are often used as mass filters, measurement of a full mass spectrum requires scanning (of voltages) and is relatively time consuming.

Experiments are often performed on pulsed molecular beams, usage of a ToF-MS allows rapid and full mass analysis of a single ion pulse.

Mass spectrometric characterization

mzeB

Mass spectrometers:mass analyzermass filtersion traps

Approaches for size-selectivity in cluster studies:a) Mass selection, accumulation, spectroscopyb) Size-specific detection ( Action spectroscopy)

3. Probing the structures

We like to understand, and to explain, observed facts in terms of structure.

Linus Carl Pauling

(1901-1994)Nobel Prize in Chemistry 1954

“The place of Chemistry in the Integration of the Sciences”, Main Currents in Modern Thought, 1950, 7, 110

CLUSTERSTRUCTURE

Ion Mobility

Anion PESTrapped Ion

ElectronDiffraction

RamanSpectroscopy

Infrared Multiple Photon Dissociation Spectroscopy

Chemical probe method

Experimental methods for structure determination of clusters

Vibrationalspectroscopies

Theory

Ligand molecules are brought into reaction with a clusterComplexes of the cluster with one or more ligands are formed depending on PL and T via consecutive reactions

saturation numbers

The chemical probe method

X + L XL + L XL2 + L … XLsat

plot (average) saturation number as function of P,plateaus indicate stable complexes

The chemical probe method

5

8

6

E.K. Parks, et al., The structure of nickel-iron clusters probed by adsorption of molecular nitrogen.Chem. Phys. 262 (2000) 151.

Ion chromatography

• The collision cross section is a measure of size (number of atoms) and shape of a cluster

rotationally averaged collision cross sections:spherical < oblate < prolate

Mass selected ions are pulled through a collision gas (He) by a weak electric field leading to a resulting drift velocity: vd = K·E

Mobility K in the gas is related tothe collision cross section

Ion mobility measurements

12

1632

TkNq

UtLK

B

More compact structures have higher mobilities

Comparison with collision cross sections for various isomers from theory geometric structure

source: bowers.chem.ucsb.edu/theory_analysis/ion-mobility

IMS-MS: a commercial technique

Waters Synapt-G2 HDMS

Si cations and anions

R.R. Hudgins, M. Imai, M.F. Jarrold, P. Dugourd, J. Chem. Phys. 111 (1999) 7865.

prolate

oblate?

‘more spherical’

Several families of cluster structuresSimilar transition size from prolate to oblate structures

Electron diffraction of trapped cluster ions

wave - particle duality

de Broglie wavelength on electrons:

12 pm for Ekin=10 keV

Electron diffraction of trapped cluster ions

mass selection, trapping, thermalization~107 ions per cm3

40 keV e-beam, ~µA currentJ.H. Parks, X. Xing in The Chemical Physics of Solid Surfaces,

Vol. 12 Atomic Clusters. (2007) 377.

Overview: TIED of anionic gold clusters

Total scattering intensity shows little size specific features use of reduced molecular intensity

Gold clusters, some example structures from TIED

observation of 2D and 3D isomer for Au12-: size for 2D/3D transition for anionic Au clusters

Anion photoelectron spectroscopy (Photoemission)

• Anions can be mass selected• Excitation energies are within the UV-vis range

• Electron affinity: vertical EA > adiabatic EA

Ekin=h-EB

Measurement of photoelectron spectra

K.H. Meiwes-Broer, Appl. Phys. A 55 (1992) 430-441.

1. Production of cluster anions2. Mass selection3. Photo excitation with vis/UV Laser4. Measure kinetic energies of electrons

Anion Photoelectron Spectroscopy of Au20-

simulation

Au20: minimum in EA (2.75 eV)

A-X separation = energy to reach firstexcited state in the neutral

≈ HOMO-LUMO gap

J. Li, X. Li, H.-J. Zhai, L.-S. Wang, Science 299 (2003) 864.

“… Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation.”

Structure and bonding of Au20-

large HOMO-LUMO gap: sign of stability

1.77 eV in Au20 vs. 1.57 eV in C6020 e: magic shell closing

5d10 are localized6s1 form 4-center-2-electron bonds (10x)

D.Y. Zubarev, A.I. Boldyrev, J. Phys. Chem. A 113 (2008) 866.

Isomerism in gold clusters

Isomer identification by:Ion chromatography (different cross section)Electron diffraction (different atomic positions)Chemical reactions (different reactivity)

Example: using O2 to remove contribution of more reactive isomers of Au10

- to anion photoelectron spectrum

W. Huang, L.-S. Wang, Phys. Chem. Chem. Phys. 11 (2009) 2663.

Origin of vibrational spectroscopy

1800 discovery of “invisible Rays of the Sun” by W. Herschel

1905 Coblentz: “Investigations of Infrared Spectra” (120 organic compounds)

1920/30’s Foundations of theoretical molecular spectroscopy

1928 Discovery of the Raman effect 1940’s structure of penicillin from group

frequencies

R.N.Jones Can. J. Spectr. 26 (1981) 1

Vibrational spectroscopy

Infrared absorption

Raman scattering

=0

=1

h h h h(

h h(

virtual state

=0

=1

h=E=1-E=0

Rayleigh-S. Raman-S.(Stokes)

Raman-S.(anti-Stokes)

Selection rules for vibrational transitions

Infrared absorption

0

eqq

Selection rules for vibrational transitions

Infrared absorption

Raman scattering

0

eqq

0

eqq

s as

IR spectroscopy of clusters in molecular beams

Not sensitive enough (low particle density)Not species specific (cluster distribution)

Absorption spectroscopy

Action Spectroscopy:More sensitive and selective: Mass spectrometric detection of absorption

Changes of the charge state (ionization)Changes of particle mass (dissociation)

An intense and tunable IR source is needed for the excitation

σnleII 0

σFeNN 0

“fingerprint” region(M-M)

IR photo dissociation of most systems requires absorption of multiple IR photons

Chemisorption energies: 1-3 eVBinding energies in transition metal clusters 3-6 eVPhysisorption energies <0.1 eV

(X-H)(C=O)

DFM / OPO CO2

Dissociation of rare gas complexes:Messenger technique

Free Electron Lasers as source of IR radiation

Wavelength depends on:kinetic energy of the electronsUndulator period u

magnetic field (~K)

2

2

2 1)2

1(2 cm

EK

e

U

The Free Electron Laser for Infrared eXperiments (FELIX)FOM Institute for Plasma Physics “Rijnhuizen”, Nieuwegein, The Netherlands

15-45 MeV electron beamtunable between 40-2400 cm-1

(up to ~3700 cm-1 on 3rd harmonic)

up to 100 mJ per macropulse (1010 W/cm2 in a micropulse)

bandwidth typically 0.5-2 %of the central wavelength

Magnetism in small rhodium clusters

Y.-C. Bae et al. Phys. Rev. B 72 (2005) 125427.

108 9

► Cubic growth can explain magnetic properties

► Eight-center bonding through d orbitals

1312A.J. Cox et al. Phys. Rev. B 49 (1994) 12295.

Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes

IR multiple photon excitation spectroscopy

Internal vibrational redistribution thermal heating

IR excitation

action

Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes

IR: 205 cm-1

depletion spectrum

frequency

inte

nsity

frequency

IR absorption spectrum

cros

s se

ctio

n resonant absorption Fragmentation of the Ar complexes

The cubic structures of rhodium clusters

Rh8 cube, Oh symmetry 1 IR active mode (t1u)

108 9 1312

e

b2

bicapped octahedral structure as identified also for other transition metals

0

+0.18 eV

+0.56 eV

+0.92 eV

Assignment of the structure of Rh8+

J. Chem. Phys. 132 (2010) 011101.J. Chem. Phys. 133 (2010) 214304.

Infrared spectroscopy of metal cluster complexes

ligand modes500-3500 cm-1 (0.06-0.43 eV)

internal cluster modes< 500 cm-1 (0.06 eV)

Structure of “bare” metal clusters

Exploring the cluster’s surface chemistry

CO at Rhn+: Size dependence of the binding site

Binding to each additional M atom leads to a shift of the C-O stetch of about 100-150 cm-1 tolower frequencies.

Observation of CO bound in 3-fold facecapping (µ3), 2-fold bridging (µ2), and linear (µ1)geometries

JACS 125 (2003) 15716J. Phys. Chem. B 108 (2004) 14591

M() CO(5)donation

M() CO(2)back donation

Assignment3.1 Rh4(CO)12 has the structure shown to the right. For the cation we

measured the infrared spectrum plotted below. What can you say about the structure of Rh4(CO)12

+?

3.2 Make suggestions for the structures of Rh3(CO)9+ and Ru3(CO)12

+ based on their given IR spectra. Both are actually very similar.Hint: Rh2(CO)8 shown below follows the 18 e valence electron rule for each metal atom, where CO is a 2e donor ligand and metal-metal single bonds are counted to contribute with one extra electron to each metal center. This gives in total 2x9 (from Rh) + 8x2 (from CO) + 2 (1 Rh-Rh bond) = 36 valence electrons, or 18 per Rh atom.For the trimers only Ru3(CO)12

+ obeys this rule for each metal atom, so, first figure out how many metal-metal single bonds are in this cluster.

Physical and chemical properties of small clusters (<100 atoms) are often strongly size-dependent

Model and reference systems

Investigation under (close to) collision free conditions

Mass spectrometry: Cluster size separation vs. sizeselective detection (action spectroscopy)

Variation of size (n), composition (n/m) and charge (z): MnLm

z

Cluster-size specific methods for characterizationAdsorption probes Ion mobility spectrometryTrapped ion electron diffractionAnion photo electron spectroscopyInfrared spectroscopy

Summary