using expression for controller name(ng-controller) in a directive in AngularJS

Instead of specifying controller name using ng-controllerI want to specify controller name via an attribute in the directive and then replace it in the template(I want to do it this weird way just to explore AngularJS).

Here is the link to the plunk: http://plnkr.co/edit/KZgLQ6?p=preview

e.g. of what I am trying to do

<sample-drtv ctrl-name="drtvCtrl"></sample-drtv>

template(sampleDrtv.html):

<div ng-controller="{{ctrlName}}"> 
  <p>{{ptagcontent}}</p>  
</div>

Directive code:

var myModule = angular.module('ng');

myModule.directive('sampleDrtv',[function(){
    return {
        restrict: "E",
            replace: true,
            templateUrl: "sampleDrtv.html",
            scope: {},
            controller: function($scope,$element,$attrs){
                $scope.ctrlName = $attrs.ctrlName;
                console.log($scope);
            }
        };

}]);

function drtvCtrl($scope){
  $scope.ptagcontent = "AngularJS Rocks";  
}

I am getting this error on my console:

Error: Argument '{{ctrlName}}' is not a function, got undefined

Any ideas on what should I do so that ctrlName is replaced in the expression? I am sure I do not understand concepts of directives completely and so I am doing a mistake?