Designing and creating the ultimate x-

ray diffractometer

By Robert Morien under supervision of Prof. Paul Lyman

REU PROGRAM

SUMMER 2004

Outline

UWM’s x-ray machine

What is x-ray radiation and what is it used for

Types of x-ray diffractometers

Kappa reconfiguration

Diffraction pattern of Si

Pentacene used for OTFT’s

X-ray diffractometry is used for:

A non-destructive technique in analyzing semi-conductor

wafers and thin films for contamination and atomic

spacings

Determination of substrate and film coherence

Information concerning the stresses and strains

between lattice and film mismatches

Primary method for determining molecular structure of

proteins, particularly DNA which was determined by use

of x-ray diffraction

Analysis of crystalline phases present in any sample

How are x-ray photons developed?

X-rays are developed in an evacuated x-ray tube

The tube is made of two electrodes, the anode and the cathode

The cathode is usually constructed of a tungsten filament held at a high negative potential

The anode is held at ground potential

The cathode is heated producing thermionic emission

Electrons are accelerated towards the anode ejecting electrons of the anode material

The hole of the ejected electron is filled by an outer shell electron which creates x-ray radiation – the x-ray source

Energy and thus the wavelength of the x-ray beam is dependent upon material selected for the anode

Types of x-ray diffractometers

Picker 2 circle 2 axes of rotation

limited only to powder diffraction

Standard four circle with eularian cradle 4 axes of rotation

Can diffract powders or crystals

Kappa 4 axes of rotation

Allows better access to sample (no obstructing chi circle)

4 circle with eularian cradle

Kappa

Kappa under construction

Addition of the fifth circle will allow an additional degree of

freedom to detect out-of-surface plane scattering vectors

Can use additional constraint to use grazing incidence

angles for surface diffraction

Diffraction pattern for Si powder I ran on the 4-

circle Eularian X-ray diffractometer during REU

x-ray diffraction pattern

0

200

400

600

800

0 20 40 60

two-theta

c.p.

s.

 Detector

d1 @ 1.28° = 34.06Ǻ = d

d2 @ 14.1° = 3.16Ǻ = d111

d3 @ 17.95° = 2.5Ǻ = d012

d4 @ 23.6° = 1.92Ǻ = d022

a

dhkl =

aSi = 5.4305Ǻ

222 lkh

a

1.54λαk Ǻ

Pentacene is a promising molecule for growth on crystalline

substrates.

Currently we are growing pentacene crystals at UWM using

molecular beam epitaxy (MBE). Paul Lyman will take these crystals

with him as he leaves for Sabbatical at CERN to determine its

crystalline structure using synchrotron radiation

What can pentacene give us?

Biodegradable and easy to reproduce

Able to operate as a transistor at room temperature

High charge mobilitys and on/off ratios

Low cost/Easy to manufacture

Organic electronics Transistors

Light emmiting diodes for display screens

Flexible