Shown in Figure 3.27 are the STM images of ordered adenine chiral chains along the (1, 2) and (-1, 2) directions obtained after annealing the sample at 490 K. These chains are in good agreement with the previous experimental findings revealed by Chen et al. [1] except that they show better intramolecular contrast of the individual
features. As seen from the
row along the (-1, 2) direction seem to be more clearly resolved than th (1, 2) direction. Since
maintained constant during scanning, we attribute the internal contrast difference in both chains to the effect of different scanning direction with respect to the molecular orientations rather than to the
molecular resolution of both chains, the tip has to be rotated at an angle with respect to the scanning direction.
In Figure 3.27b, a sudden change in
corresponds to the variations in the electronic conductivity of the tunnelling junction. We suggest that this phenomenon is due to the variation of the tip condition during scanning; for example, an adenine molecule
can modify the contrast of the same features within the same image. With an adenine adsorbed on the W tip, the tunnelling junction will become Cu
rather than Cu-adenine-W. In this new junction, a higher resistance could be responsible for the depressed molecular features in the image [1]. Hence, we should emphasize that this depression is due to an electronic effect rather than a height effect.
Fig. 3.27:STM images of adenine dimer chains with different contrast at medium coverage (0.20 n 0.96 V, 33 × 33 nm2, 0.20 nA,
direction is different from that along (
molecular rows with respect to the substrate indicate
seen from the image, Figure 3.27a, individual elongated 1, 2) direction seem to be more clearly resolved than th
Since the tip apex structure and tip-sample bias voltage are maintained constant during scanning, we attribute the internal contrast difference in both chains to the effect of different scanning direction with respect to the molecular to the asymmetry of tip structures. In order to achieve both chains, the tip has to be rotated at an angle with respect
sudden change in the contrast of the adsorbates
onds to the variations in the electronic conductivity of the tunnelling junction. phenomenon is due to the variation of the tip condition during
an adenine molecule can be adsorbed on the STM tip, y the contrast of the same features within the same image. With an adenine adsorbed on the W tip, the tunnelling junction will become Cu-adenine
W. In this new junction, a higher resistance could be essed molecular features in the image [1]. Hence, we should emphasize that this depression is due to an electronic effect rather than a height effect.
images of adenine dimer chains with different contrast at medium coverage (0.20 n , 0.20 nA, -0.86 V, 33 × 33 nm2). a) The contrast of the chains along the (1, 2) direction is different from that along (-1, 2) direction. b) The sudden change in the contrast of
rows with respect to the substrate indicates a variation in the tunnelling junction.
elongated features of the 1, 2) direction seem to be more clearly resolved than those along the sample bias voltage are maintained constant during scanning, we attribute the internal contrast difference in both chains to the effect of different scanning direction with respect to the molecular In order to achieve both chains, the tip has to be rotated at an angle with respect
of the adsorbates is observed, it onds to the variations in the electronic conductivity of the tunnelling junction. phenomenon is due to the variation of the tip condition during can be adsorbed on the STM tip, which y the contrast of the same features within the same image. With an adenine adenine-adenine-W W. In this new junction, a higher resistance could be essed molecular features in the image [1]. Hence, we should emphasize that this depression is due to an electronic effect rather than a height effect.
images of adenine dimer chains with different contrast at medium coverage (0.20 nA, - a) The contrast of the chains along the (1, 2) in the contrast of the unnelling junction.
A molecular resolution STM image of the dimer chains aligning along (1, 2) and (- 1, 2) directions is shown in Figure 3.28; the slight contrast difference of the imaged molecular features between the two chains is identified. The chains along the (1, 2) direction formed by two parallel rows of circular shape spots arranged side by side, while the molecular features appear of elongated shape in the chains along the (-1, 2) direction. Even though the observation of contrast difference between the adsorbate features, the periodicities of the unit cell in both chains are very close. The unit cell is about 7.5 Å in length and has C2symmetry. Since the coexisting molecular chains are
mirror related and each is homochiral, the observation of slight contrast discrepancy of the adsorbate features could result from a scanning drift or the effect of the fast scanning direction with respect to the specific molecular orientation in both domains. Note that the contrast difference of the individual molecular features forming the mirror related chains is not distinguished in STM when scanning across surface areas of over 400 Å; only arrays of bright and elongated features are observed in both chains.
Corresponding structural models, denoted A5A5, A6A6 and Ā5Ā5, Ā6Ā6, are
presented in Figure 3.28, following the structural model suggested by Chenet al.[1]. In the superimposed model, the dimer,A5A5orĀ5Ā5, is connected with dimers of the
same chirality by the second type H-bond site 6 along the chain growth direction. Along each chain, only molecules of the same chirality are included and the chain growth direction is strictly related to the specific molecular chirality. The molecular planes are oriented nearly parallel to the surface and all the amino nitrogen atoms are allocated at preferential on-top sites.
Fig. 3.28: Molecular resolution proposed structural models and
14 nm2). Each adenine row is composed of adenine dimers of along the chain growth direction
substrate at on-top sites.