CHINESE JOURNAL OF PHYSICS
VOL. 43, NO. 1-II
FEBRUARY 2005
Chemisorption of Methyl (CH3 ) and Methylnitrene (NCH3 ) Radicals on Cu
surfaces studied by STM and LEED
Woei Wu Pai,1 Yuet Loy Chan,1 and Tung J. Chuang1, 2
1
Center for Condensed Matter Sciences,
National Taiwan University, Taipei 106, Taiwan, R.O.C.
2
Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei 106, Taiwan, R.O.C.
We report direct STM observation of the formation of two-dimensional islands of methyl
radical (CH3 ) on Cu(111) and Cu(110) surfaces, and methylnitrene radical (NCH3 ) islands
on Cu(110). In corroboration with LEED measurements, plausible adsorption models of
each system are proposed. In all studied cases, first-order reaction kinetics of long-chain
alkene production from methyl or azomethane (CH3 N2 CH3 ) production from methylnitrene
can be correlated with the adsorbate islanding behavior.
PACS numbers: 68.37.Ef, 68.43.Fg, 82.30.Cf
I. INTRODUCTION
Methyl (CH3 ) and methylnitrene (NCH3 ) radicals have been considered critical intermediates in many hydrocarbon catalytic reactions [1, 2]. The identification of these
species under regular high pressure reaction conditions is rather difficult. Instead, many
studies investigate the adsorption and reaction behavior of the radicals on single-crystal
metal surfaces under ultrahigh vacuum (UHV) condition in order to understand the basic
reaction steps and mechanisms [1]. In recent years, we have studied CH3 on Cu(110) and
Cu(111) [3–7], and NCH3 on Cu(110) [8] with a variety of techniques including temperature
program desorption (TPD), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), low energy electron diffraction (LEED), and high-resolution
electron energy loss spectroscopy (HREELS). One of the most interesting finding is that
chemisorbed CH3 radicals can readily react by thermal activation to form long-chain hydrocarbons via a sequence of chain-propagation reactions, all following the first order reaction kinetics [5,6]. Namely, the reaction scheme can be represented by CH3 (ads) →
CH2 (ads)+H(ads), followed by CH3 (ads)+CH2 (ads) → C2 H5 (ads) → C2 H4 (g)↑+H(ads),
C2 H5 (ads)+CH2 (ads) → C3 H7 (ads) → C3 H6 (g)↑+H(ads), etc. chain reactions. Desorption kinetics of methylnitrene on Cu(110) has also been studied [8], showing also first-order
reaction kinetics. These reaction schemes are supported by the product distributions detected in TPD spectroscopy. It is surprising to observe that a series of high mass alkene
products (from C2 H4 to as high as C5 H10 in some instances) are generated from CH3 at the
same temperature and independent of surface coverages. Similarly, simple recombination of
NCH3 followed by first-order desorption kinetics of azomethane is also counter-intuitive. It
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c 2005 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA
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was suggested by our group that this is very likely a consequence of adsorbate aggregation
at even only a small fraction of saturation coverage. This islanding behavior can explain
why the reactants can overcome time-consuming diffusion steps that will inevitably lead to
high-order reaction kinetics; because they do not adsorb far apart on the surface, even at
small coverages, due to adsorbate islanding.
Such islanding phenomena was recently reported for CH3 on Cu(111) with scanning
tunneling microscopy (STM) [3]. Here we review the island formation of CH3 on Cu(111)
and report new observation of direct islanding behavior for CH3 and NCH3 on Cu(110) with
STM and LEED experiments. In all cases, we obtained STM images with clear molecular
resolution that exhibit structures consistent with the LEED measurements. In particular,
STM measurements shed additional insight on the adsorption structure and allow us to
propose plausible adsorption models.
II. EXPERIMENTS
Experiments were conducted with a variable-temperature VT-STM (Omicron) housed
in a home-built ultrahigh vacuum (UHV) chamber with a reverse-view LEED apparatus and
gas dosing facility. The UHV system has a base pressure < 3×10−11 torr. The Cu(111) and
Cu(110) crystals were cleaned with repeated sputter-anneal cycles (Ne+ , 1000 eV, 870 K).
Methyl radicals were generated by pyrolysis of azomethane (CH3 NNCH3 ) in a nozzle source
with a procedure described previously [4]. Methylnitrene radicals on Cu(110) were obtained
by dissociation of adsorbed azomethane molecules at room temperature [8]. Optimal dosing
conditions for these adsorbate systems were described previously and are listed in Table 1.
For STM imaging of methyl, the samples were cooled to below ∼90 K. For STM imaging
of methylnitrene, the sample was held at room temperature. Sample temperature was
measured with a Si diode in direct thermal contact with the crystals and is precise within
±3 K.
TABLE I: Dosing conditions for studied adsorbate systems.
Adsorbate system Source gas pyrolysis Generated gas Substrate Temp.
NCH3 /Cu(110)
CH3 N2 CH3
No
CH3 N2 CH3
300 K
CH3 /Cu(111)
CH3 N2 CH3
Yes
CH3
300–400 K
CH3 /Cu(110)
CH3 N2 CH3
Yes
CH3
∼400 K
III. RESULTS AND DISCUSSION
√
√
Fig. 1(a) depicts the ( 3 × 3)-R30◦ LEED pattern of CH3 (ads) on Cu(111) at a
coverage of Θ= 0.3 ML and a substrate temperature of 210 K. Here 1 ML refers to the
saturation coverage as one monolayer. This structure is in complete agreement with STM
image as shown in Fig. 1(c). With this adsorbate structure, the two-dimensional density
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FIG. 1: LEED pattern of CH3 on Cu(111) taken at 210 K. CH3 coverage Θ = 0.3 ML. Beam
energy = 67 eV. (b) An STM image showing irregularly shaped CH3 islands. Sample size: 400
nm × 400 nm. Imaging condition: sample bias VS = 1.57 V, tunneling current It= 0.38 nA. CH3
exposure = 0.36 L, 1L = 10−6 torr-sec. Substrate temperature = 58 K. (c) An STM micrograph
taken at 50 K. Sample size: 15 nm × 15 nm. Imaging condition: VS = 0.77 V, It= 0.1 nA. Note
that CH3 island boundary appears fluctuating, and the terrace is covered by diffusing CH3 species
which
cause
random excursion in heights of STM scanning traces. (d) An adsorption model of the
√
√
( 3 × 3)-R30◦ structure of CH3 /Cu(111).
ratio between CH3 (ads) adlayer and top Cu atoms is 1: 3. When the clean Cu(111) is
exposed to a smaller amount of CH3 below saturation coverage, the chemisorbed radicals
readily form segregated two-dimensional islands with various sizes and somewhat irregular
shapes, as shown in Fig. 1(b). The STM analysis was always conducted at low temperature
(30–90 K). This is because clear molecule-resolved STM images were difficult to obtain, and
islands were much more mobile and could be perturbed by STM tip at higher temperatures.
From the photoelectron diffraction (PhD) analysis [9] and theoretical modeling [10–
12], the radicals most likely reside on the three-fold hollow sites. Calculations show that
hcp is the preferred site [10,11]. However, the energy difference between the fcc and the
hcp site adsorption is small (< 25 meV/per CH3 ) and is within calculation uncertainty
[10]. Experimentally, PhD analysis indicates that fcc site is preferred, however. From
STM experiments, we cannot exclude the possibility of simultaneous occupation of fcc and
hcp sites. The placement of H atoms is also of great interest. Calculations show that H
atoms prefer to orient toward Cu atoms, as depicted in Fig. 1(d). In fact, there is a direct
interaction between H and Cu atoms that leads to a rotational barrier of at least ∼ 170
meV [11]. At our measurement temperature (>50 K), CH3 rotation is possibly deactivated.
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WOEI WU PAI, YUET LOY CHAN, AND TUNG J. CHUANG
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FIG. 2: (a) LEED pattern of CH3 on Cu(110) taken at 300 K. Θ = 0.5 ML. Beam energy = 67 eV.
(b) An STM image showing aggregated CH3 islands. Sample size: 335 nm × 335 nm. VS = 0.41 V,
It=0.53 nA. Substrate temperature= 44 K. CH3 exposure = 0.032 L. (c) An STM image of c(2×2)
CH3 islands with molecular resolution. Substrate temperature = 44 K. Sample size: 6.6 nm × 11.3
nm. Imaging condition: VS = 0.57 V, It= 0.44 nA. In contrast with CH3 islands on Cu(111), here
the CH3 island boundary appears static, and no sign of diffusing CH3 species on terrace is observed.
(d) An adsorption model of the c(2×2) structure of CH3 /Cu(110). See text for details.
However, we have not observed internal structure of adsorbed CH3 .
Methyl radicals adsorbed on Cu(110) form a c(2×2) structure as shown in the LEED
pattern of Fig. 2(a). As for CH3 on Cu(111), methyl radicals also form islands on Cu(110),
and such islands are most readily observed at cryogenic temperature. Fig. 2(b) shows
an STM image of these CH3 islands. Interestingly, the islands do not seem to reflect
the anisotropic nature of an fcc(110) surface. Most islands appear more or less isotropic,
but fractal-like, shapes with no clear indication of island shape anisotropy. In contrast
with CH3 islands on Cu(111), c(2×2) islands of CH3 on Cu(110) is not fully close-packed.
Furthermore, the CH3 island boundary is almost static at a temperature of 50 K. This is
contrasted by the fluctuating boundary of CH3 islands on Cu(111) (cf. Fig. 1(c)). Similarly,
in CH3 /Cu(111) the Cu terraces are covered with a significant number of diffusing CH3
species. This CH3 “fluid” phase is not observed on Cu(110). Fig. 2(c) illustrates the
static island periphery and the lack of diffusing CH3 on terraces. Clearly, CH3 diffusion on
Cu(110) must be slower than that on Cu(111).
One interesting question is on the adsorption site of CH3 on Cu(110). On Cu(111),
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FIG. 3: (a) LEED pattern of a saturated NCH3 layer on Cu(110) taken at room temperature. Beam
energy = 80 eV. (b) An STM image of NCH3 islands with anisotropic shapes. Sample size: 300 nm
× 300 nm. Imaging condition: VS = 0.33 V, It= 0.53 nA. (c) A molecule-resolved STM image of the
“zig-zag” type NCH3 island. Sample size: 12 nm × 12 nm. Imaging condition: VS = 0.03 V, It= 0.13
nA. (d) A molecule-resolved STM image of the “rectangle” type NCH3 islands. Sample size: 9 nm
× 9 nm. Imaging condition: VS = 0.75 V, It= 0.47 nA. Note that two types of NCH3 radicals with
different contrasts are observed for this structure. (e) An adsorption model of the p(3×2) structure
of NCH3 /Cu(110). Both the “zig-zag” and “rectangle” structures have p(3×2) unit cells.
CH3 is shown to adsorb on three-fold hollow sites, both from theoretical calculations and
photoelectron diffraction experiments. Importantly, H atoms strongly prefer to adsorbed
atop Cu atoms (“H-atop” geometry). Can CH3 adsorb on such three-fold and H-atop
geometry on Cu(110)? Indeed, it is possible because the open troughs along the closepacked < 110 > direction are bounded by < 111 > microfacets. These microfacets could
provide a local bonding geometry for CH3 similar to that on Cu(111). This is depicted by
the lower right two unit cells of Fig. 2(d). Two possible three-fold hollow sites, i.e., fcc
and hcp, are shown. In fact, we have observed adjacent CH3 island domains shifted by
approximately half unit cell (not shown). This is consistent with our proposed model. If
one assumes that CH3 are adsorbed on atop or two-fold hollow sites in the Cu troughs,
as shown in the two upper left unit cells of Fig. 2(d), the presence of two CH3 domains
indicates that both the atop and two-fold hollow sites are occupied. This appears unlikely.
Azomethane molecules on Cu(110) readily decompose into methylnitrene radicals
upon adsorption at room temperature [8]. Previous TPD experiments show that NCH3
desorbed at ∼375 K as azomethane, and the reaction kinetics is first-order with a desorp-
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WOEI WU PAI, YUET LOY CHAN, AND TUNG J. CHUANG
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tion temperature independent of NCH3 coverage. The LEED pattern shown in Fig. 3(a)
indicates a p(3×2) adsorption structure. STM images taken at room temperature reveal
anisotropically shaped NCH3 islands with their elongated direction perpendicular to the
Cu close-packed < 110 > direction (Fig. 3(b)). This is because that the p(3×2) structure
has a looser packing along < 110 > and the NCH3 island boundary along this direction will
have a higher step energy and thus a shortened length. Closer STM inspection of the NCH3
islands show that each p(3×2) unit cell contains two NCH3 radicals. This is not unexpected
because if NCH3 is adsorbed with N bonding to the substrate, the packing of NCH3 should
be restrained by the van der Waal (vdW) diameter of a CH3 . A p(3×2) unit cell containing
two NCH3 presumably optimizes such vdW interactions. Another very interesting phenomenon is that there are two coexisting adsorption structures as shown in Figs. 3(c), (d).
The structure of Fig. 3(c) is a zig-zag structure with no clear contrast between each NCH3 ,
and the structure of Fig. 3(d) is a rectangle structure showing a clear contrast between
two types of adsorbed NCH3 . At room temperature, we have observed both structures
but with a slight preference toward the zig-zag one. Occasionally, these two structures can
suddenly switch in an STM micrograph, and the transition appears abrupt during the <
0.1 second time interval of a single STM scan line (not shown). This indicates that these
two structures must be quite similar and may involve only slight distortion of NCH3 lattice
positions. We propose an adsorption model in which radicals occupy the four corners of a
p(3×2) unit cell and the other radical occupies either the central or offset sites inside the
unit cell. We have tentatively placed NCH3 on the Cu two-fold bridge sites in Fig. 3(e).
Adsorption of NCH3 at hollow sites along the Cu troughs is also possible. Fig. 3(e) is
consistent with STM images showing NCH3 reside along the < 110 > Cu atomic rows (not
shown here), and preliminary results of quantum chemistry calculations [13]. The zig-zag
structure is depicted by the left p(3×2) unit cell in which all NCH3 adsorbed on equivalent
sites. The rectangle structure is shown by the right p(3×2) unit cell in which there are
two types of equally populated NCH3 adsorbed at different sites. Simultaneous presence of
these two structures and a slight preference toward the zig-zag structure indicate that they
are similar in energy and the rectangle structure is metastable.
At present, we have conducted extensive ab-initio theoretical calculations for CH3 on
Cu(111) [10]. Similar calculations are not yet available for CH3 and NCH3 on Cu(110).
To provide a fuller justification of our proposed models, further calculations are being
conducted and will be published elsewhere.
IV. CONCLUSION
In summary, we have conducted high-resolution STM studies for two important radical species, i.e., methyl (CH3 ) and methylnitrene (NCH3 ) on Cu surfaces. The most
prominent chemisorption characteristic in all cases is that adsorbates aggregate into islands
even at small coverages. Such islanding could provide a ready supply of reactants during
surface chemical reactions; this is relevant to the observed first-order reaction kinetics in the
formation of long-chain alkene from CH3 on Cu(111) and Cu(110), and azomethane from
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CHEMISORPTION OF METHYL (CH3 ) . . .
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NCH3 on Cu(110).
based on STM observation, we propose plausible adsorption
√ Finally,
√
models for the ( 3 × 3)-R30◦ structure of CH3 on Cu(111), the c(2×2) structure of CH3
on Cu(110), and the p(3×2) structure of NCH3 on Cu(110).
Acknowledgments
The authors wish to thank the National Science Council and Ministry of Education
of R.O.C. for support of this work.
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