graphite rod surface coated with c 60 paul dunk o oo oo o o o o oo oo o o o o oo oo o o o oo oo o o...
TRANSCRIPT
Graphite rod surface coated with C60
Paul Dunk
OOO
OOO
OO
OOO
OOO
OO
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
OOO
OOO
OO OO
OOO
O OOO
OOO
OO OO
OOO
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
OOO
OOO
OO OO
OOO
O
OOO
OOO
OO OO
OOO
OOO
OOO
OO OO
OOO
O
Carbon atoms the hot (10000o) focal point C60 evapourates unscathed from surrounding warm domain
C60 on graphite rod – little fragmentationsmall Cn species are ingested by C60 forming larger fullerenes
m/z1,2001,000800600400200
450
400
350
300
250
200
150
100
50 C70
Desorbed C60
<2% fragmentation of C60
50% of the C60 converted to
higher fullerenes
200 400 600 800 1000
m/z1,000950900850800750
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
MS of C60+n(neven) fullerenes
13C enrichedCarbon + C60
reaction
C62
C64
C66
C68
C70
C76
C74
C72
C78 C80
m/z1,6001,4001,2001,000800600400200
350
300
250
200
150
100
50
C60 coated on a glass rod – just C60
C60
Typical desorbed C60
mass spectrum. No growth
m/z1,000950900850800750
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Expanded view of larger C60 + Cn fullerenes
The larger fullerenes become increasingly 13C enriched. In agreement with small Cn species inserting into C60 and higher fullerenes to form even higher fullerenes
C62
C64
C66
C68
C70
C76
C74
C72
C78 C80
Note C62 – C68 are not stable
Fullerenes grow by ingestion of small carbon species
Cn + C2 → Cn+2 etc
The rod translated and rotated by stepper motor
laser pulse vaporisesrod
Helium pulse C60 reacts with C, C2 exits and undergo supersonic expansion
C60 coatedgraphite rod
Cluster Source
Helium
Pulsed Nozzle
Rod coated with 13C enriched amorphous carbon (ca 10%)
and pure C60
Pulsed Laser
Helium
Pulsed Nozzle
Pulsed Laser
Rotating/translating rod coated with 13C enriched amorphous carbon (ca 10%) and pure C60
carbon plasma cluster beam
Laser and graphite disk
skimmer hole to mass spectrometer
Laser Vapourisation Cluster Beam System - Smalley
The very sharp edged skimmer skims the expanding pulse into a very narrow beam
Supersonic Expansion into the vacuum chamber cools the clusters to very low
temperatures
Skimmer action more slowly
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
2. The atoms at the plasma/He interface cool to form small linear species: C2 C3, C4 etc
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
2. The atoms at the plasma/He interface cool to form small linear species: C2 C3, C4 etc
3. Monocylic rings and at least two other families of carbon molecules form
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
2. The atoms at the plasma/He interface cool to form small linear species: C2 C3, C4 etc
3. Monocylic rings and at least two other families of carbon molecules form
4. Small fullerene cages from C28… onwards are created
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
2. The atoms at the plasma/He interface cool to form small linear species: C2 C3, C4 etc
3. Monocylic rings and at least two other families of carbon molecules form
4. Small fullerene cages from C28… onwards are created
5. Small fullerenes grow into larger cages by ingestion
Multi stage formation mechanism
1. Laser fires and produces a ca10,000K plasma of C atoms
2. The atoms at the plasma/He interface cool to form small linear species: C2 C3, C4 etc
3. Monocylic rings and at least two other families of carbon molecules form
4. Small fullerene cages from C28… onwards are created
5. Small fullerenes grow into larger cages by ingestion
6. Final stage condensation to solid product – only Cn n=60, 70 and higher n species survive
accumulation octopole
target rod skimmer
transfer octopole
ICR cell
stepper motor
pulsed valve
10-7 torr
diffusion pump
10-7
turbo pump
10-8
turbo pump
10-10
turbo pump
pulsed valve
source
supersonic expansion
skimmed into beam
C60 reacts with carbon species
The pulses pass across the chamber at about 10 Hz
…and the signal integrated perhaps 100 to a1000 or more times
The University of Sussex machine
ions accumulated in octopole
3-10 laser shot accumulated
transferred to the ICR cell
supersonic expansion
skimmed into beam
C60 reacts with carbon species
ions accumulated in octopole
3-10 laser shot accumulated
transferred to the ICR cell
m/z1,000950900850800750700650600550
500
450
400
350
300
250
200
150
100
50
C60 + amorphous 13C coated on a quartz rod
Desorbed C60
C60 ingests 13C species
C60 + Cn → C62 C64 C66 C68 C70 C72 etc
This study shows unequivocally that fullerenes can grow by ingestion of smaller carbon species
in this case
with Paul Dunk and Alan Marshall
Multi stage formation mechanism
Multi stage formation mechanism
Refinement of a closed cage growth mechanism proposed by Heath for the fullerenes and Endo and Kroto for nanotube growth
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
2. The atoms at the plasma/He interface cool to form C2 C3, C4 etc
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
2. The atoms at the plasma/He interface cool to form C2 C3, C4 etc
3. Monocylic rings (and another family) form
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
2. The atoms at the plasma/He interface cool to form C2 C3, C4 etc
3. Monocylic rings (and another family) form4. Small fullerene cages C28… are created
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
2. The atoms at the plasma/He interface cool to form C2 C3, C4 etc
3. Monocylic rings (and another family) form4. Small fullerene cages C28… are created
5. Small fullerenes grow into larger cages by ingestion
Multi stage formation mechanism
1. Laser fires and produces a ca10000o plasma of C atoms
2. The atoms at the plasma/He interface cool to form C2 C3, C4 etc
3. Monocylic rings (and another family) form4. Small fullerene cages C28… are created
5. Small fullerenes grow into larger cages by ingestion6. Final stage – only Cn n=60, 70 and higher n species
survive
Graphite rod surface coated with C60
The pulses pass across the chamber at about 10 Hz
…and the signal integrated perhaps 100 to a1000 or more times
The University of Sussex machine
The rod translated and rotated by stepper motor
laser pulse vaporisesrod
Valve emits a helium pulse
Fullerenes react with carbon vapor (C, C2) in the “clustering zone”. Then, the gas exits the channel and undergoes a supersonic expansion to create a cooled, molecular beam
fullerene-coatedgraphite target rod
Cluster Source
m/z727726725724723722721720719718717
500
450
400
350
300
250
200
150
100
50
Expanded view of desorbed C60 – normal isotope distribution
Normal ratio
← 13C12C59 ~ 60% of 12C60 signal
12C60 signal
One strongly held conjecture was that C60 and C70 would be cul-de-sacs and our results indicate that this conjecture is incorrect.
One strongly held conjecture was that C60 and C70 would be cul-de-sacs and our results indicate that this conjecture is incorrect.
Our results indicate that the primary nascent distribution of fullerenes shows almost no evidence of IPR stabilisation
which is a surprise to at least meRequiring a final stage which non IPR cages do
not survive
One strongly held conjecture was that C60 and C70 would be cul-de-sacs and our results indicate that this conjecture is incorrect.
Our results indicate that the primary nascent distribution of fullerenes shows almost no evidence of IPR stabilisation
m/z1,000950900850800750700650600550
500
450
400
350
300
250
200
150
100
50
C60 + amorphous 13C on a quartz rod
Desorbed C60
Q: How many pulses do you need to accumulate?A: A single laser shot is used vaporize the target during a single Helium pulse. Ten singe laser shot + He pulse are used to accumulate ions.
Q:How do you decide when to transfer them.A: After the final laser shot, a voltage at the “back” of the accumulation octopole switched, and the ions are transferred to the ICR cell. The switching of the voltage is controlled by the computer program interface.
Q: How many runs do you need in general for an average result?A: 3 time-domain aquisitions are averaged for when “growing” a preformed fullerene…….the signal is extremely strong.And 10 time-domain acquisitions are averaged when form endohedrals from a graphite-metal target. Thus, up to 10 time-domain acquisitions are averaged.
Answer to questions from email.
Fullerenes react with carbon vapor in the “clustering zone”, then the gas exits the channel and undergoes a supersonic expansion. As the clusters move from a region of high pressure through a small orifice into a high vacuum, they undergo a supersonic expansion. The random thermal energy of the clusters is converted into a directed motion (creating a cooled, molecular beam in which very few collisions occur) toward the skimmer and the ions subsequently enter the ion optics where they are accumulated and then transferred to the ICR cell for detection.
Q: How do you stop the pulse of ions in the accumulation trap A: The ions are confined radially by an oscilating radiofrequency within in octopole, and axially by voltages at the ends of the “accumulation octopole:.
Answer to questions from email.
…undergo supersonic expansion and are skimmed into beam
10-7 torrdiffusion pump
10-7
turbo pump
10-8
turbo pump
10-10
turbo pump
Fullerenes react with
carbon species in the
vapourisation zone, then exit
reaction channel
the ions which enter the ion optics, where they are acuumulated in the central octopole segment
After 3-10 single laser shot accumulations, the ions are transferred to the ICR cell, which is located within in the bore of a 9.4 tesla superconducting magnet. Under the influence of the high magnetic field, the ions exhibit cyclotron motion. The ions induce a current on electrodes, which is detected as an “image current” in the time domain, and then the signal is converted to the frequency domain by an FT. Thus, the mass of the ion is detected as a frequency.
m/z1,000950900850800750
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Expanded view of larger C60 + Cn fullerenes
Larger fullerenes increasingly 13C enriched
C62
C64
C66
C68
C70
C76
C74
C72
C78 C80
2.7 eV
C28
2.4 eV
C284-
+ C2
facileC30 and higher
C28 C284-
Less reactive
C30 and higher
Electron donation stabilizes small fullerenes – prevents addition of small Cn
+ C2
Added electron density at the triple pentagon junction stabilizes – prevents C2 addition
Empty cage
endohedral
Fullerene Growth Mechanism
• “fullerene road” – Small fullerenes are the first to form, and then growth to larger fullerenes occur by uptake of small carbon species such as C2.
• There has been no evidence that fullerene growth can actually occur this way…until now!
• This growth model accounts for all experimental observations, for endohedrals and empty cages. This is potentially extremely significant if this checks out.
C60
• C60 has been desorbed many times and analyzed many times by mass spec.
• No growth to larger fullerenes occur.
• However, most of these experiments are performed in a vacuum under conditions where fullerene growth will not be significant.
Bulk C60 coated on a graphite rod
• The rod was put in our cluster source
• Importantly, the experiment was performed exactly as I would if I were ablating a “clean” carbon rod to produce fullerenes.
• Pulse gas, laser timing, etc all known to be the conditions to see fullerenes
• The result: Addition of C2 to form larger fullerenes!
m/z1,2001,000800600400200
450
400
350
300
250
200
150
100
50
Small carbon clusters added to C60 to form larger fullerenes. The coated C60 did not significantly fragment, but did significantly add carbon to form larger fullerenes.
C70
Desorbed C60
Very minor fragmentation of C60
Significant growth to larger fullerenes
m/z1,2001,000800600400200
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
Larger clusters are FULLERENES. C70 (formed from desorged C60) was SWIFT isolated, and then subjected to collision with He while exciting (SORI).
These larger clusters are clearly fullerenes as C2 fragmentation occurs.
C70
C68C60
C60 coated on a quartz rod
• To gain further insight, C60 was coated on a quartz rod
• The same experiment was performed to see if fullerene growth occurred.
• If no growth occurred, the small carbon species from the graphite were likely adding to C60 in the coated graphite rod experiment.
• The result: no growth!
C60 + amorphous 13C on quartz rod
• C60 was mixed was some amorphous 13C. There was much more C60 than amorphous 13C, approximately 3:1.
• This mixture was applied to a quartz rod(from toluene)
• Growth to larger fullerene occurred, and they were more 13C enriched with size. This is consistent with small cluster addition to C60
585580575570
585580575570
m/z1,000950900850800750700650600
m/z1,000950900850800750700650600
m/z590585580575570
m/z585580575570
U@Cn+ directly formed from
U/graphite target enriched with amorphous 13C --for a total 13C content of 10%
U@C28 experimental
U@C28 simulated
U@C28 experimental
U@C28 (9.5% 13C) simulated
U@C28
U@C44
U@C36
U@C28
U@C36
U@C44
BOTTOM-UP GROWTH
U@Cn+ directly formed from
U/graphite target
Growth mechanism and endoherals
• It is shown through our experiments that the classical endohedrals of Sc, La, and now Ti, Hf, Zr, U all strongly form endohedral fullerenes. But fullerenes smaller than C60 are most abundant, a clear deviation from the empty cages.
• Ionic model – electrons from the metal are transferred to the carbon cage in endohedral fullerenes giving, essentially, an indissociable salt...the cage is negatively charged, the encapsulating metal is positively charged. Our experiments coupled with the theoretical data show this principle applies to small fullerenes too.
• Electron transfer from the encapsulating metal to fullerene cage stabilizes the small fullerene from small carbon addition to larger fullerenes. This is why metals that can donate 3-4 electrons to the cage predominately form smaller fullerenes.
Growth mechanism and M@C28
• Our experiments show that M@C28 forms first. And then larger clusters are seen under conditions that allow more growth.
• The metal nucleates initial growth.
• Experiments will need to be performed with coating a rod with an endohedral, I plan to ask Shinohara for a sample to use. This will prove that the “fullerene road” applies to endohedrals as well as the empty cages.
• Only a tetravalent metal can stabilize C28 sufficiently to yield a M4+@C284-
• Our calculations show that the donated electrons reside at the most reactive triple pentagon junction…the end result: C28 does not completely react to larger fullerenes.