Phonon Coupling - Electrons in CdSe, CdS Cores, and Shell Quantum Dots

Tech 2023-05-25 04:26:17 Source: Network

Xu Chenyuan's StudyEditor | Xu Chenyuan's StudyWe found that peak CdS frequency affects excitationStrong dependence on energyBetween the experimental frequency of the CdSe+CdS combination band and the sum of the corresponding fundamental frequencyThere is still a huge differenceThis indicates that the main transition at high excitation energy is limited to the CdSe core or CdS shell, and therefore cannot enhance the binding band transition between the core and shellThe Raman intensity ratio of CdS to CdSe further supports this viewpoint. The Phonon of CdSeLO in the Lowest Exciton TransitionElectron phonon coupling is slightly weaker in core/shell structures than in pure CdSe quantum dotsThis is contrary to the expectation of the Frehlich coupling mechanism

Xu Chenyuan's Study

Editor | Xu Chenyuan's Study

We found that peak CdS frequency affects excitationStrong dependence on energyBetween the experimental frequency of the CdSe+CdS combination band and the sum of the corresponding fundamental frequencyThere is still a huge difference

This indicates that the main transition at high excitation energy is limited to the CdSe core or CdS shell, and therefore cannot enhance the binding band transition between the core and shellThe Raman intensity ratio of CdS to CdSe further supports this viewpoint. The Phonon of CdSeLO in the Lowest Exciton TransitionElectron phonon coupling is slightly weaker in core/shell structures than in pure CdSe quantum dotsThis is contrary to the expectation of the Frehlich coupling mechanism. Therefore, we will discuss the possible explanations for this difference.

Compared to single component nanocrystals, core/shell structures exhibit greater diversity in spectral, photophysical, and chemical properties, and their synthetic definitionsClear methods for core/shell structures have been developed

(/)CdSe/CdSLow temperature electron spectroscopy on single nanocrystals reveals resolved phonon modesIts strength gives a 1S-1S exciton jumpElectron phonon coupling strength of transition

Raman spectroscopy is mainly used to study the alloying phenomenon and lattice mismatch at the CdSe/CdS interfaceThe presence of lattice strain in the core and/or shell caused by-(EPC)Raman excitation profile and absolute scattering cross-section/

LO207/285-300LOLO

The appearance of this combination strip indicates that,Resonant exciton transitions involving both cadmium selenide nuclei and cadmium sulfide shellsCdSe/CdSLO600Except at the highest excitation energyLO0.1-0.2-0.2This is slightly lower than the LO overtone intensity observed in pure cadmium selenide NCs

The Frequency of Raman Lines in Core/Shell Structures and Resonance ConditionsThere is a significant dependency1CdSe/CdSThe frequency of cadmium selenide increases with the increase of shell thickness3-4cm-1

The LO frequency of cadmium sulfide shows more significant changes,Added up to 15 centimeters due to excitation being tuned to shorter wavelengthsCdSe+CdS5-20

by comparison,Overtones are clearly observed4Because a fixed excitation wavelength is usedLO624CdSe/CdS/

Cadmium selenide LO has shifted towards a higher frequency by 3-7 centimetersBetween bare cadmium selenide core and thick cadmium sulfide shellCdSe/CdS/>three hundred and fifty0cm-1

These two basic principles areThe lowest exciton peak has been enhancedAt higher energies, the cadmium sulfide band exhibits additional enhancement

In the resonance Raman process, the Raman shift indicates the ground state phonon frequency, and the Raman shift of a specific transition is not dependent onResonance conditionLO

The biggest change isLOThe comparison of thin shell and thick shell spectra indicates that these changesCannot be explained by the heterogeneity of the sample3cm-1.12-1

Thick quantum dots typicallyHas a high phonon frequency of cadmium sulfide

For CdSe+CdS combined frequency bandsThe significant difference between frequency and the sum of two fundamental frequencies/The anharmonicity demonstrated by overtone frequency is quite smallLO~1%LOVery large anharmonic coupling

CdSe/CdS/Pure cadmium sulfide nanocrystals formed by uniform nucleation during shell growthLOBut it will not produce any combined bands with cadmium selenide phonons

514-458nm/LOThese frequencies are enhanced by resonance of different exciton transitions

Due to the resonance Raman intensity of the combined band, two modes are required to pass throughEnhanced by the same resonant transitionThis mechanism only allows specific CdSe/CdS modes to be displayed as combination bands, which may differ from the average value of the basic principleA considerable number of different frequencies

In fact, the width and asymmetry of the "LO phonon" of cadmium sulfide clearly indicate that it isConsisting of multiple normal modesThe Raman activity of specific phonon modes is determined by the spatial overlap between phonons and resonant excitons1S-1S

The 1S electron and hole wave function (especially the hole) has a strong peak at the center of the particle, andThe increase in shell radius results in a decrease in densityby comparison,Resonance or near resonance with many different excitons excited by shorter wavelengths/Core phonon mode and shell phonon mode

Near the cadmium selenide interface, the lattice mismatch between the two materials tends toExpand cadmium sulfide latticeLeading to the significant contribution of CdS type modes to atoms near the interfaceReduced vibration frequencyCdS/CdSeLO15-25

The higher frequency cadmium sulfide mode is generated in the shell region that is less affected by the lattice strain caused by the interface (the peak Raman frequency displayed by two pure cadmium sulfide quantum dots and bulk cadmium sulfide is 300-305cm at room temperature),Near 300-301 centimeters, we measured at high excitation energy)

Although the strain mechanism is certainly effective, calculations of elastic continuum indicate thatThis effect is too small to explain all the excitation wavelength dependence of cadmium sulfide LO phonons

These calculations are consistent with the qualitative concept, that is, if lattice strain is the only factor determining these frequency shifts, then the shift of the cadmium selenide mode towards higher frequencies should be approximately equal toDisplacement of cadmium sulfide towards lower frequencies(~140C)The alloying at the core/shell interface should be minimal

Thin shell, uneven width=880cm-1
Energy/ cm-1	Homog. width/cm-1	Degree/A	S (cadmium selenide, 203cm-1)	S (cadmium selenide, 209cm-1)	S (cadmium sulfide) , 285cm-1)	S (cadmium sulfide) , 300cm-1)
seventeen thousand seven hundred and thirty	three hundred and fifty	three	0	zero point one two five	zero point zero three	0
nineteen thousand four hundred and fifty	eight hundred and fifty	two point seven five	zero point two four	0	zero point zero three	0
twenty thousand seven hundred and fifty	one thousand	three point one five	zero point one eight	0	zero point zero eight	0
twenty-two thousand	one thousand	one point seven	zero point nine eight	0	028	0
twenty-two thousand and three hundred	one thousand	two point six	0	0	0	zero point four
twenty-three thousand and one hundred	three thousand	six point five	0	0	0	0

However, even on physically sharp interfaces,The lattice of nuclear and shell materials will also be distortedThe phonon modes on the interface may involve the mixing of selenium and S motion,Thus appearing at a lower frequency than pure cadmium sulfide phononsThe lower frequency is due to mode mixingOverall lower frequency phonon

LOThe high-frequency cadmium selenide mode is only coupled to the lowest energy exciton(1Se1S3/2)LO


Thick shell, uneven width=750cm-1
Energy/ cm-1	Homog. width/cm-1	Degree/A	S (cadmium selenide, 203cm-1)	S (cadmium selenide, 211cm-1)	S (cadmium sulfide) , 285cm-1)	S (cadmium sulfide) , 300cm-1)
seventeen thousand and thirty	three hundred and fifty	three point two nine	0	zero point one two five	zero point zero eight	0
eighteen thousand and seven hundred	eight hundred and fifty	three5	zero point three two	0	008	zero point zero four five
nineteen thousand nine hundred and fifty	one thousand	three point five	zero point one two five	0	zero point zero eight	zero point zero four five
twenty-one thousand and seven hundred	one thousand	three	zero point one eight	0	zero point five	0
twenty-two thousand	one thousand	three	0	0	0	two
twenty-two thousand and six hundred	three thousand	1zero point five	0	0	0	0

The results indicate that most of the absorption occurs in> 4000cm-1 is above the lowest exciton absorption band, which is basically limited toTransition of Cadmium Selenide Core or Cadmium Sulfide Shell-The viewpoint on the length scale has been proposed for quantum dots 52 and PbSe53 in lead sulfideCdSe/CdS

The bulk band shift and extensive experimental evidence indicate that in CdSe/CdS core/shell quantum dots, the lowest exciton transition involves a hole, which is largelyLimited to cadmium selenide core and electron equivalent delocalization throughout the core and shell(III)There is only a small amount of spatial separation between electrons and holesThe lowest exciton state of the core/shell structure should involve a considerable internal electric field, therefore, stronger coupling polarityOptical phonons pass through the Frolich mechanism

/-CdSe/ZnSe/Not that electrons extend into the shell,

But another main mechanism of EPC in semiconductors is deformation potential coupling, which isA short-term effectVolume change has strong Frank Canton activityThe volume change situation describes allChanges in bandgap energy during uniform expansion or compression of bonds

It can be seen that there is no net volume change related to the motion along the optical phonon coordinates, and this mechanism should beA negligible source of EPCStrong size dependence of EPC

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Tag: Phonon Coupling Electrons in CdSe CdS Cores and Shell