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Absolute configuration assignment of molecules and crystals in discussion
Volume 19, number
2
ABSOLUTE
CHEMICAL PHYSICS LETTERS
CONFIGURATION
ASSIGNMENT
OF MOLECULES
15 bfarch 1973
AND CRYSTALS
IN DISCUSSION H.H. BRONGERSMA and P.M. MUL Philips Research Laboratories. Eindhorlen, The Neti~erlandr Received 22 Deccmbcr
1972
The absolute configurations of asymmetric molecules and polar crystals have been based on measurements of the anomalous X-ray scattering. Recent theoretical investigations of Tanaka suggest that these measurements have been misinterpreted, impIying that the assignment of all absolute configurations should be changed into their antipodes. The incorrectness of this suggestion is experimentally proven by studying the opposite (polar) faces of noncentre symmetric crystals such as ZnS and CdS by nobk gas ion reflection mass spcctrometry WIRMS).
The absolute configurations of molecules and crystals have always played an important r61e in organic and inorganic chemistry. As early as 1874 van ‘t Hoff [I] and le Be1 [2] introduced the concept taining an asymmetric
of antipodes for molecules concarbon atom. Although these
antipodes show a different optical rotary power, it was impossible to determine the relation between specific configuration and direction of the rotary power. The convention of Fischer [3], however, gained general acceptance [4] . Sixty years later Bijvoet and coworkers [5,6] ?using anomalous X-ray scattering, showed that the Fischer convention happens to be in agreement with reality. The same technique had earlier been applied to a similar problem encountered in inorganic chemistry concerning noncentrosymmetric crystals. A wellknown example of such a structure is ZnS. For such a ZnS crystal the lattice can be regarded as being built up from double layers in the ( 111) direction, consisting of a Zn plane and an S plane at a distance of only a quarter of the lattice constant (fig. 1). This leads to the well-known polar character of a ZnS crystal. On one of the { 111) faces the top layer will consist of only Zn atoms, whereas S atoms will constitute the top layer of the opposite face. A direct consequence of such a polar structure is the different chemical behaviour for the two faces (differences in etching and crystal growth).
/ Zn
-5 -/
Zn
‘S L/
Zn
-5
Fig. 1. A ZnS crystal can be regarded as being built up from double layers perpendicular to the ( 111) direction.
Coster et al. [7] used the anomalous X-ray scattering to differentiate between the two opposite faces of ZnS. Warekois et al. [8,9] applied the same analysis to various III-V (InAs) and II-VI (ZnS, CdS, etc.)
compounds. The X-ray results were correlated to etching behaviour of the surfaces so that simple etching tests are now available to identify the surfaces. Knowledge of the crystaliographic polarity is important, e.g., in solid-state electronics, since this polarity influences the band bending near the surface and thus such properties as photoemission [IO] . Very recently Tanaka [l 1) reinvestigated the anomalous X-ray scattering using quantum fieId theory. His work throws doubt on the correctness of the commonly-used classical theory. According to Tanaka, the sign of the scattering vectors or alternatively.the atomic.scattering factors shouid be corrected in 21-7
Volume 19, number 2
the anomalous X-ray theory. This would imply that the absolute,configuration &signments of asymmetric molecules should be altered into the antipodes of the presently accepted configurations. Such a change would seriously influence the correspondence between theory and experiment in techniques such as circular dichroism. The same alteration would be necessary for the polar crystals. Since it is quite unusua1 that classical and quantum mechanics should give the opposite sign for a pheno menon, an independent experimental answer is sought. Lcw-energy ion scattering from the polar faces of noncentrosymmetric crystals was chosen to test the theory. This technique has been applied before to the polar faces of a CdS crystal by Smith [ 12) and by Efremenkova et al. [ 131, Smith, using specular reflection of 2000 eV He+ ions and a total scattering angle of 90”, observed a difference between the two faces. However, on both faces the spectrum was dominated by scattering from Cd atoms. Efremenkova et al., using the same conditions, found more Cd on the (OOOi) face than on the (0001) face. However, when the angle of incidence was lowered to 20” with the surface and the incident energy increased to 4 keV, more Cd was detected on the (0001) face. Computer calculations [ 131 for the specular reflection of 2 keV He+ ions from non-rearranged poIar surfaces of CdS indicate that only Cd should be observed at the Cd face, while only S should be detected at the sulphur face. The discrepancy between theory and experiment may be due to re&ngements of the surfaces, or possibly to the high intensities of the ion beams ( 10m3- lOA A/cmZ) or to the surface preparation. Recently [14, IS] we were able to demonstrate that the scattering of ions from a surface can be used to se!ecGvely mass analyse the outermost atomic layer of this surface. A clear advantage of the technique is that the interpretrnion of the results is very straightforward. Moreover, the relative intensities for the various atoms are quite comparable (atoms with the lower atomic number having often a somewhat highei sensitivity .[I51 ). Noble gas ions which are back-scattered from a surface lose an amount of energy which is characteristic for the mass of the surfAce atom with which they,
218
..
‘-
15 hiarch 1973
CHEMICAL PHYSICS LETTERS
800
600 Energy
of
Scattered
1000
ions ieVj_
I+ =. 2. The two opposite { 1I I} faces of a ZnS crystal are analysed by ion scattering. Specular reflection of 1000 eV NC+ ions is used; the total scattering angle is 45’. The drawn line is the spectrum for the (1 11) face. The relative intensities of the curies are arbitrary.
collide. Since collision times are much shorter than vibration times, the interaction of the atom with the lattice can be neglected. He+ or Ne’ ions with impact energies of the order of 1 keV are often chosen to obtain a high neutralization probability for the ions. Since only ionized scattered particles can be detected, it is unlikely that doubly or multiply scattered particles would be observed. This is especially true for the He+ ions. A simple two-body collision model is then sufficient to describe the interaction of an ion with a solid target [i4]. If we consider an ion with an incident energy Ei and majsMion which is scattered over an angle 0 by a target cf mass Mat, the final. energy of the ion will be: Ef
=
sin2/j)‘/2 ZE iy 1+-Y
COSB+(y*-
I
(1)
Volume 19, number 2
15 hfsrch 1973
CHEhlICAL PHYSICS LETTERS
where y = Iw,tIMi,, and is assumed >l. This expression is simply derived by solving the equations for conservation of momentum and energy. The electronic configuration and state of the atom have no influence on the energy loss of the ion. Since Eiand Mi,, are kept constant during a given experiment, the energy spectrum of the back-scattered ions is essentially a mass spectrum. Ln fig. 2 such mass spectra are shown for the two opposite {I I1 } faces of ZnS. The surfaces are nnalysed by specular reflection of Ne+ ions with an incident energy of 1000 eV. The total scattering angle is 4S” and the azimuth is the (I 120) direction. An incident beam of 7 X lo-* A is used which is focussed to a spot of about 0.01 cm2. The crystal was cleaned with acetone and chemically polished for 10 min at 95°C in a 0.5 IM solution of potassium dichromate in sulphuric acid. At the same time this etch allows one to identify the faces according to the normaily excepted convention: shallow dishes are formed on the (111) surface [9]. Adsorbed layers due to the interaction of the crystal with the atmosphere are first removed by sputtering. The measurements are performed in ultra high vacuum. For singly scattered Ne+ ions the theoretical values (1) are 830 and 679 eV for scattering from a Zn or S atom respectively.--- The spectra show practically only sulphur on the (I 11) face, while practically no sulphur is detected on the opposite (111) face. On the (11 I) face the major peak is due to scattering from Zn atoms, while the small peak is due to Ne+ ions which reach the andyser after being scattered by two Zn atoms. The theoretical energy for the ions after such a double collision is simply obtained by applying eq. (1) twice. For Ne+ ions which scattered twice over 22.5”, the predicted fiial energy is 908 eV. This is in good agreement with experiment. The energy of the small peak is so high, that this peak cannot be due to sulphur-sulphur or zinc-sulphur collisions (maximum energies of 825 and 88 1 eV respectively). Clearly the (Ill) surface is the zinc face and the (?ii) surface the sulphur face. This designation of the faces is the same as the result from the old classical theory for anomalous X-ray scattering. We have also reached this conclusion in studies of the polar faces of CdS and GaP [ 15;. Fig. 3 shows the spectra for the two opposite faces of CdS. He+
-Cd
Fig. 3. The two opposite {OOOI}faces of 8 CdS crysta1 are analysed by ion scattering. Specular reflection of 2000 eV He+ ions is used; the total scattering angle is 9Oa. The drawn line is the spectrum for the (0001) face. The relative intensities of the curves are arbitrary.
ions of 2000 eV and a scattering angle of 90” are used for the analysis. An incident beam of 1 X IOwi A is used which is focussed to a spot of about 0.01 cm2. The ions are in the (I 12, azimuth. Prior to the measurements the crystat was cleaned with acetone etched for 30 set in a 2% solution of HCI in water. The peaks agree nicely with the theoretical value (1) of 1862 and 1556 eV for Cd and S respectively. Again the difference between the polar faces is clearly demonstrated. The spectra arc very different for the two faces. On one side only suIphur atoms could be detected, while the cadmium peak is the prominent feature on the opposite face. Other experiments on CdS where the azimuth and angle of incidence were varied (keeping the total scattering angle constant) gave a similar result. In agreement with the theoretical predictions by Efremenkova et al. [13], the difference between the polar faces is clearly demonstrated. TIzis also indicates that the non-rearranged surface structure is a good approximation for these faces. The polarity of the crystal was determined by anomalous X-ray scattering using the reflection of continuous background [Is]. In order to correlate rhese results also with the Xray analysis of Warekois et al. [9], we etched the crystal according to their procedure: two minutes in a ,
:
219
2
ABSOLUTE
CHEMICAL PHYSICS LETTERS
CONFIGURATION
ASSIGNMENT
OF MOLECULES
15 bfarch 1973
AND CRYSTALS
IN DISCUSSION H.H. BRONGERSMA and P.M. MUL Philips Research Laboratories. Eindhorlen, The Neti~erlandr Received 22 Deccmbcr
1972
The absolute configurations of asymmetric molecules and polar crystals have been based on measurements of the anomalous X-ray scattering. Recent theoretical investigations of Tanaka suggest that these measurements have been misinterpreted, impIying that the assignment of all absolute configurations should be changed into their antipodes. The incorrectness of this suggestion is experimentally proven by studying the opposite (polar) faces of noncentre symmetric crystals such as ZnS and CdS by nobk gas ion reflection mass spcctrometry WIRMS).
The absolute configurations of molecules and crystals have always played an important r61e in organic and inorganic chemistry. As early as 1874 van ‘t Hoff [I] and le Be1 [2] introduced the concept taining an asymmetric
of antipodes for molecules concarbon atom. Although these
antipodes show a different optical rotary power, it was impossible to determine the relation between specific configuration and direction of the rotary power. The convention of Fischer [3], however, gained general acceptance [4] . Sixty years later Bijvoet and coworkers [5,6] ?using anomalous X-ray scattering, showed that the Fischer convention happens to be in agreement with reality. The same technique had earlier been applied to a similar problem encountered in inorganic chemistry concerning noncentrosymmetric crystals. A wellknown example of such a structure is ZnS. For such a ZnS crystal the lattice can be regarded as being built up from double layers in the ( 111) direction, consisting of a Zn plane and an S plane at a distance of only a quarter of the lattice constant (fig. 1). This leads to the well-known polar character of a ZnS crystal. On one of the { 111) faces the top layer will consist of only Zn atoms, whereas S atoms will constitute the top layer of the opposite face. A direct consequence of such a polar structure is the different chemical behaviour for the two faces (differences in etching and crystal growth).
/ Zn
-5 -/
Zn
‘S L/
Zn
-5
Fig. 1. A ZnS crystal can be regarded as being built up from double layers perpendicular to the ( 111) direction.
Coster et al. [7] used the anomalous X-ray scattering to differentiate between the two opposite faces of ZnS. Warekois et al. [8,9] applied the same analysis to various III-V (InAs) and II-VI (ZnS, CdS, etc.)
compounds. The X-ray results were correlated to etching behaviour of the surfaces so that simple etching tests are now available to identify the surfaces. Knowledge of the crystaliographic polarity is important, e.g., in solid-state electronics, since this polarity influences the band bending near the surface and thus such properties as photoemission [IO] . Very recently Tanaka [l 1) reinvestigated the anomalous X-ray scattering using quantum fieId theory. His work throws doubt on the correctness of the commonly-used classical theory. According to Tanaka, the sign of the scattering vectors or alternatively.the atomic.scattering factors shouid be corrected in 21-7
Volume 19, number 2
the anomalous X-ray theory. This would imply that the absolute,configuration &signments of asymmetric molecules should be altered into the antipodes of the presently accepted configurations. Such a change would seriously influence the correspondence between theory and experiment in techniques such as circular dichroism. The same alteration would be necessary for the polar crystals. Since it is quite unusua1 that classical and quantum mechanics should give the opposite sign for a pheno menon, an independent experimental answer is sought. Lcw-energy ion scattering from the polar faces of noncentrosymmetric crystals was chosen to test the theory. This technique has been applied before to the polar faces of a CdS crystal by Smith [ 12) and by Efremenkova et al. [ 131, Smith, using specular reflection of 2000 eV He+ ions and a total scattering angle of 90”, observed a difference between the two faces. However, on both faces the spectrum was dominated by scattering from Cd atoms. Efremenkova et al., using the same conditions, found more Cd on the (OOOi) face than on the (0001) face. However, when the angle of incidence was lowered to 20” with the surface and the incident energy increased to 4 keV, more Cd was detected on the (0001) face. Computer calculations [ 131 for the specular reflection of 2 keV He+ ions from non-rearranged poIar surfaces of CdS indicate that only Cd should be observed at the Cd face, while only S should be detected at the sulphur face. The discrepancy between theory and experiment may be due to re&ngements of the surfaces, or possibly to the high intensities of the ion beams ( 10m3- lOA A/cmZ) or to the surface preparation. Recently [14, IS] we were able to demonstrate that the scattering of ions from a surface can be used to se!ecGvely mass analyse the outermost atomic layer of this surface. A clear advantage of the technique is that the interpretrnion of the results is very straightforward. Moreover, the relative intensities for the various atoms are quite comparable (atoms with the lower atomic number having often a somewhat highei sensitivity .[I51 ). Noble gas ions which are back-scattered from a surface lose an amount of energy which is characteristic for the mass of the surfAce atom with which they,
218
..
‘-
15 hiarch 1973
CHEMICAL PHYSICS LETTERS
800
600 Energy
of
Scattered
1000
ions ieVj_
I+ =. 2. The two opposite { 1I I} faces of a ZnS crystal are analysed by ion scattering. Specular reflection of 1000 eV NC+ ions is used; the total scattering angle is 45’. The drawn line is the spectrum for the (1 11) face. The relative intensities of the curies are arbitrary.
collide. Since collision times are much shorter than vibration times, the interaction of the atom with the lattice can be neglected. He+ or Ne’ ions with impact energies of the order of 1 keV are often chosen to obtain a high neutralization probability for the ions. Since only ionized scattered particles can be detected, it is unlikely that doubly or multiply scattered particles would be observed. This is especially true for the He+ ions. A simple two-body collision model is then sufficient to describe the interaction of an ion with a solid target [i4]. If we consider an ion with an incident energy Ei and majsMion which is scattered over an angle 0 by a target cf mass Mat, the final. energy of the ion will be: Ef
=
sin2/j)‘/2 ZE iy 1+-Y
COSB+(y*-
I
(1)
Volume 19, number 2
15 hfsrch 1973
CHEhlICAL PHYSICS LETTERS
where y = Iw,tIMi,, and is assumed >l. This expression is simply derived by solving the equations for conservation of momentum and energy. The electronic configuration and state of the atom have no influence on the energy loss of the ion. Since Eiand Mi,, are kept constant during a given experiment, the energy spectrum of the back-scattered ions is essentially a mass spectrum. Ln fig. 2 such mass spectra are shown for the two opposite {I I1 } faces of ZnS. The surfaces are nnalysed by specular reflection of Ne+ ions with an incident energy of 1000 eV. The total scattering angle is 4S” and the azimuth is the (I 120) direction. An incident beam of 7 X lo-* A is used which is focussed to a spot of about 0.01 cm2. The crystal was cleaned with acetone and chemically polished for 10 min at 95°C in a 0.5 IM solution of potassium dichromate in sulphuric acid. At the same time this etch allows one to identify the faces according to the normaily excepted convention: shallow dishes are formed on the (111) surface [9]. Adsorbed layers due to the interaction of the crystal with the atmosphere are first removed by sputtering. The measurements are performed in ultra high vacuum. For singly scattered Ne+ ions the theoretical values (1) are 830 and 679 eV for scattering from a Zn or S atom respectively.--- The spectra show practically only sulphur on the (I 11) face, while practically no sulphur is detected on the opposite (111) face. On the (11 I) face the major peak is due to scattering from Zn atoms, while the small peak is due to Ne+ ions which reach the andyser after being scattered by two Zn atoms. The theoretical energy for the ions after such a double collision is simply obtained by applying eq. (1) twice. For Ne+ ions which scattered twice over 22.5”, the predicted fiial energy is 908 eV. This is in good agreement with experiment. The energy of the small peak is so high, that this peak cannot be due to sulphur-sulphur or zinc-sulphur collisions (maximum energies of 825 and 88 1 eV respectively). Clearly the (Ill) surface is the zinc face and the (?ii) surface the sulphur face. This designation of the faces is the same as the result from the old classical theory for anomalous X-ray scattering. We have also reached this conclusion in studies of the polar faces of CdS and GaP [ 15;. Fig. 3 shows the spectra for the two opposite faces of CdS. He+
-Cd
Fig. 3. The two opposite {OOOI}faces of 8 CdS crysta1 are analysed by ion scattering. Specular reflection of 2000 eV He+ ions is used; the total scattering angle is 9Oa. The drawn line is the spectrum for the (0001) face. The relative intensities of the curves are arbitrary.
ions of 2000 eV and a scattering angle of 90” are used for the analysis. An incident beam of 1 X IOwi A is used which is focussed to a spot of about 0.01 cm2. The ions are in the (I 12, azimuth. Prior to the measurements the crystat was cleaned with acetone etched for 30 set in a 2% solution of HCI in water. The peaks agree nicely with the theoretical value (1) of 1862 and 1556 eV for Cd and S respectively. Again the difference between the polar faces is clearly demonstrated. The spectra arc very different for the two faces. On one side only suIphur atoms could be detected, while the cadmium peak is the prominent feature on the opposite face. Other experiments on CdS where the azimuth and angle of incidence were varied (keeping the total scattering angle constant) gave a similar result. In agreement with the theoretical predictions by Efremenkova et al. [13], the difference between the polar faces is clearly demonstrated. TIzis also indicates that the non-rearranged surface structure is a good approximation for these faces. The polarity of the crystal was determined by anomalous X-ray scattering using the reflection of continuous background [Is]. In order to correlate rhese results also with the Xray analysis of Warekois et al. [9], we etched the crystal according to their procedure: two minutes in a ,
:
219